The "Mod Installation" part of this guide is likely what most people will be here to see, however the later parts of this guide are more like a Build-a-Modpack. Looking to make Subnautica feel difficult and/or absolutely terrifying again? Well this guide will walk you through my favorite mods to make that happen. If you are just starting out modding and like the idea of "difficult and/or absolutely terrifying", give it a thorough read and you will likely end up with a fun version of my Modpack tailored just for you by the end.

Even if difficult and/or terrifying doesn't sound appealing to you, there will still be mods that have excellent QoL tweaks you may not know about, so give it a skim over.

Now, since this is all out of date; we're going to need to swap over to the Legacy version of the game to run these mods. To do that right click Subnautica in your library, hit properties, betas, then in the drop down box, select the legacy branch and wait for it to install.

1) Nexus account [users.nexusmods.com] Nexus is a sight where tons of mods for tons of games are shared. You likely already have an account if you have modded games such as Skyrim before. And you will need one to download almost everything in this guide.
2) Download QModManager [nexusmods.com]
3) Extract QModManager. (I personally use 7-Zip [7-zip.org])
4) Place the contents in the Subnautica directory. To find it, go to your library, right click Subnautica in your library, mouse over "Manage", and click "Browse Local Files".
5) Once the Mod Manager is installed all that's left is downloading and extracting the mods to the QMods folder that is now in your Subnautica directory.

- Make sure mods' .dll files and/or other files are inside of a folder called the mods name.
The directory should look like this: Subnautica\QMods\{Mod Name}\{Mod Name}.dll

Lets talk ReShade, note that this is perfectly skippable.

1) Download ReShade [reshade.me]
2) Make sure you have .NET Framework [dotnet.microsoft.com]
3) Run ReShade Setup.exe, hit Browse, select Subnautica.exe (To find it go to Steam\steamapps\common\Subnautica), after that select DirectX 10/11/12, when it asks to "Select a preset to install" hit Skip (We are going to install that ourselves), after that you can just hit Next twice and it will be finished.
4) Lastly, download a ReShade preset of your choice, extract its .ini file into your Subnautica directory (I already told you how to find it in 2 different ways), start up the game, hit the Home key to open the ReShade menu, then it will pop up the tutorial (feel free to read through or skip), Click the drop down box that says "ReShadePreset" and select the preset you downloaded.

• Subtletica SweetFX 2.0 [nexusmods.com]
  Is my personal overall favorite, to quote the mod page: "A bit more saturation but not overblown, a bit darker but not pitch black" Plus its performance friendly. I get 45 fps with and without it!
  Note that it doesn't actually require ReShade and can be manually installed into the game, however I recommend going the ReShade rout, as its ingame overlay for easy modification or swapping of your shaders on the fly is very handy. Its ReShade preset is under the optional files labeled "Subtletica 1.0.1 (ReShade Framework profile)"
  
  
• DarkNautica [nexusmods.com]
  Is my favorite option for a horror focused ReShade.
  
  
• Subnautica - True Light [nexusmods.com]
  Is my favorite option for a pretty visuals focused ReShade.

In this section I will list off tons of mod suggestions while trying to give nice & quick descriptions for them, I'm also going to use these little rated categories to better get across how I find it impacts my game. I'm going to place various categories under most of the mods and rate each of these categories with 0-5 stars. Example:
Immersion: ★★★
Difficulty: ★
Horror: ★★
QoL: ★★★★★

I suggest downloading all of the mods in this list, but feel free to skip them if you feel you don't want the QoL most of them offer.

- SMLHelper (Modding Helper) [nexusmods.com]
A prerequisite to a whole bunch of mods. To the point that its currently the 3rd most downloaded mod for Subnautica on all of nexus, just get it.

- QuickSlotsPlus [nexusmods.com]
Increases the amount of quick slots you get from 5 to 10+.
Requires: SMLHelper (Modding Helper)
QoL: ★★★★★

- SnapBuilder - Snap-to-Grid for Subnautica [nexusmods.com]
This mod makes building a million times better. "snap building" Might sound like it could be irritating at times, but that couldn't be further from the truth, SnapBuilder allows you to quickly
toggle between snap building, fine snapping, and no snapping with hotkeys.
Requires: SMLHelper (Modding Helper)
QoL: ★★★★★

- Radial tabs [nexusmods.com]
Turns the crafting menu into a more modular circular menu.
Immersion: ★★

- EasyCraft [nexusmods.com]
Automatically crafts ingredients for blueprints and allows you to craft using items from nearby storage containers.
QoL: ★★★★★

- Tweaks and Fixes [nexusmods.com]
As the name would suggest, adds a ton of tweaks and bug fixes, also a ton of options. Read the later "Tweaks and Fixes Configuration" section for info on how I like to change the game with this mods settings, but if you don't care for what I like to change with it, feel free to leave it default.
Requires: SMLHelper (Modding Helper)

- No Loud Bangs in Cyclops [nexusmods.com]
Ever get tired of passing through the safe shallows with the Cyclops and hearing BANG BOOM BANG as all the fish hit your sub? well this mod fixes that.
QoL: ★★★

- Hud Tweaks [nexusmods.com]
Adds various settings for adjusting hud elements such as the crosshair. Its base settings are great, but you can also adjust them to your liking.
Requires: SMLHelper (Modding Helper)
Immersion: ★★★★

- Running With Tools [nexusmods.com]
Ever been exploring an island either at night or in a cave, and notice that you can't hold your flashlight while sprinting? Well this mod fixes that.
QoL: ★★★

- CyclopsNearFieldSonar [nexusmods.com]
Adds the sonar display from the Seaglide to the right hand side of the Cyclops.
Immersion: ★★
QoL: ★★

I personally run all of these mods in my game, but feel free to pick and choose.

- Migrate [nexusmods.com]
Adds the real life phenomena of sea creatures swimming up and down from the depths with the day/night cycle.
I never thought I would encounter a mod I love so much yet also hate so much at the same time.
Requires: SMLHelper (Modding Helper)
Immersion: ★★★★★
Difficulty: ★★
Horror: ★★

- ExtraOptions [nexusmods.com]
I use this mod for its Water Murkiness option, I suggest setting it to 300-360 for the desired effect of reduced view distance. As a bonus, it will reduce pop-in!
Requires: SMLHelper (Modding Helper)
Immersion: ★★★
Horror: ★★★★★

- DeathRun - Roguelike Super-Hard [nexusmods.com]
DeathRun is an extremely modular mod that adds a ton of options. Read the later "DeathRun Configuration" section to learn what this mod can do. (Forget the "Super-Hard" part, its only super hard if you set it that way)
Requires: SMLHelper (Modding Helper)

- Refillable Oxygen Tanks [nexusmods.com]
This mod is very easily described as overpowered (Quadrupled Tank capacity but they must be refilled at a fabricator), however, when paired with some settings from DeathRun it can create an extremely fun experience. The settings I suggest changing are as follows:
Surface Air: Hard (Pre/Post Radiation) or Death Run (Never Breathable)
Vehicle Costs: Death Run (exotic costs)
Normally the DeathRun mod expects you to use Air Pumps to filter surface air and pipes to dive down deep enough to get the resources required for the Seamoth, but with Refillable Oxygen Tanks. It makes for a really fun adventure where you can actually dive really deep with nothing but an air tank, but its also a mad dash to get back home before your tank runs dry. Having Vehicle costs set to exotic costs will make the Seamoth cost a lot of rare materials such as ghost weed, which are not fun at all! really fun to collect with nothing but an O2 Tank.
Requires: SMLHelper (Modding Helper)
Immersion: ★★★
Difficulty: ★★★

- De-Extinction [nexusmods.com]
Adds 15 new creatures to Subnautica ranging from small to leviathan sized. Having a new leviathan type that you don't know the location of is really scary, and how dangerous it is doesn't help either. It also adds a predator sized creature called the "Twisteel", these things feel like they are everywhere and are rather dangerous.
Requires: SMLHelper (Modding Helper) and ECC Library [nexusmods.com]
Difficulty: ★★★
Horror: ★★★★

- WhiteLights [nexusmods.com]
Replaces the Flashlight and Seaglide's blue lights with a more white alternative.

- Base Legs Removal [github.com]
I use this mod to remove the legs from all the Corridor base pieces. I find it makes bases look way less cluttered having legs only on the Moonpool and Multipurpose room.
Note that this mod is not on Nexus currently, the link will take you directly to a download for it via Github.
Requires: SMLHelper (Modding Helper)

In this section I will explain what each setting does if its not already clear, and give my recommendations of what to set them to, if you want to know the real specifics of what each setting does, that information can be found at the bottom half of This document. [nexusmods.com]

Note that "Easy" always refers to Disabled for this mods settings.
Also Note that any Difficulty/Horror/Immersion ratings are based on my suggested setting, not the setting as a whole.

Settings:

• Damage Taken:
  I personally use Love Taps (up to 25% more) but I suggest just taking your pick. Setting it really high allows Reapers to destroy your Seamoth with a single grab, which adds quite a few points to the fear factor if you ask me.
  
  
• Surface Air: Whether surface air is breathable.
  I suggest Hard (Pre/Post Radiation) or Death Run (Never Breathable). And then also bringing Refillable Oxygen Tanks [nexusmods.com]
  Difficulty: ★★★
  
  
• Radiation: When the Aurora explodes and until it is repaired, Radiation will blanket the ocean down to the depth you select here.
  I suggest Death Run (60m).
  Difficulty: ★★
  
  
• Nitrogen and the Bends: Introduces a Nitrogen system that limits the speed you can ascend.
  I suggest Death Run.
  Essentially, this setting will force you to rise slowly or take damage rising quickly. There are various ways to overcome this, such as using vehicles which will protect you, or using medkits which will purge 50% of the nitrogen from your system.
  Immersion: ★★★
  Difficulty: ★★★
  
  
• Personal Diving Depth: Gives you a crush depth similar to your vehicles.
  I personally use Easy (Disabled), and have never actually screwed around with this setting. So, I cannot really give my opinion on it.
  
  
• Depth of Explosion: When the Aurora explodes, the damage will reach this deep.
  I suggest Death Run (100m) You will have to either build a base down that low, or evacuate down there for the explosion.
  Difficulty: ★★
  
  
• Explosion Time: The time before the Aurora explodes.
  I suggest adjusting this one as you go. Start with 60 or 90 mins depending on how confident you are, then if you end up below 100m with a base with enough time to spare to lower it a notch, go ahead and lower it.
  
  
• Power Costs: Increases power costs, and further increases them while in radiation. Also reduces power production.
  I personally use Easy (Disabled). I haven't ever really screwed around with this one either so, no opinion.
  
  
• Batteries: Replaces vanilla batteries with Copper Batteries that cannot be recharged, rechargeable ones must be crafted with lithium.
  I suggest Hard. Stepping it up to Death Run would probably work just fine too.
  Difficulty: ★★★
  
  
• Power to Exit Vehicles: Power consumption is based on depth.
  I personally use Easy (Disabled). And have never screwed with this setting, but after reading its specifics, it sounds interesting. Might want to give it a try.
  
  
• Vehicle Costs: Increases the cost to build Vehicles.
  I suggest Death Run (Exotic Costs), along side Refillable Oxygen Tanks [nexusmods.com]
  Exotic Costs forces you to collect various plants from around the world, and diving for them with just an O2 Tank is the most unfun thing I have ever done! very fun.
  Difficulty: ★★★
  Horror: ★★★★
  
  
• Habitat Builder: (Misnamed Apparently?) This setting actually affects the cost of all the one-time-build tools.
  I suggest Hard or Death Run.
  Difficulty: ★★★★
  
  
• Scans Required: Increases the number of scans required to unlock blueprints.
  I suggest WORSE than DeathRun. It will be unlikely a single biome or even wreck will contain enough fragments to unlock what you are after, further increasing the chances of running into something undesirable while you explore.
  I averaged the scans required & rounded them to achieve this little bit of info:
  Hard: On average 1.5x as many scans.
  Death Run: On average 2x as many scans.
  WORSE than DeathRun: On average 2.5x as many scans.
  Difficulty: ★★
  Horror: ★★★★
  
  
• Creature Aggression: Increases aggression at the 20min and 40min marks.
  I suggest Death Run. Internal Screaming Combine this with reducing the view distance using Extra Options to allow Leviathans to see further than you for a truly fun experience!
  Difficulty: ★★★
  Horror: ★★★★★
  
  
• Start Location: Allows you to randomize your starting location or choose one.
  I suggest BASIC GAME. Randomized starting location is a little too random for my taste.
  
  
• Radiation FX if Immune: Radiation effect amount.
  I suggest Aurora as Chernobyl. It forces you to carry a Flashlight while making your way through the Aurora so you can see through the insane radiation.
  Difficulty: ★★
  
  
• Show Bends and Air Warnings
  I suggest Whenever Encountered until you have an understanding of the mechanic. However if you aren't using "Nitrogen and the Bends" or have experience with the mechanic, switch it to Never.
  
  
• Don't Tip Escape Pod Over
  I suggest checking this setting, as I never really felt it added anything substantial, its just kinda annoying.
  QoL: ★★★

Tweaks and Fixes has tons of settings, so I'm not going to list all of them off. Only going to give my suggestions for what to change. If you want to know exactly what each setting does, that information can be found on the mods Nexus Page [nexusmods.com]
Feel free to make all your own personal changes.

Settings:

• Unmanned vehicles can be attacked
  I personally use: Only_if_lights_on
  
  
• New hunger system: You don't regenerate health when you are full. You don't take damage when your food or water value is 0. Your food and water values can drop to -100. When your food or water value is below 0 your movement speed will be reduced proportionally to that value ('Player movement tweaks' has to be enabled). When either your food or water value is -100 your movement speed will be reduced by 50% and you will start taking hunger damage. Your max food and max water value is 200. The higher your food value above 100 the less food you get when eating: when your food value is 110 you lose 10% of food, when it's 190 you lose 90%
  I suggest checking this.
  Immersion: ★★★★
  
  
• Eating Raw Fish
  I suggest Risky. It will make eating raw fish have a chance of giving you an equal and opposite amount of food/water.
  Immersion: ★★
  
  
• Fruit growth time: Time in days it takes a lantern tree fruit to grow.
  I suggest 2 or 3. The default being 1.
  Immersion: ★★
  Difficulty: ★
  
  
• Big creature speed multiplier: Speed of creatures that you can't catch will be multiplied by this.
  The amount you increase this is equal to the amount you hate yourself will be proportional to how difficult it will be to get away from a Leviathan that sees you.
  I personally use 1.2.
  Difficulty: ★★
  Horror: ★★★
  
  
• Disable tutorial messages: Disables messages that tell you to 'eat something', 'break limestone', etc.
  I suggest checking this.
  QoL: ★★
  
  
• Realistic oxygen Consumption: Causes oxygen consumption based on depth to increase linearly instead of sharply at depths 100m and 200m.
  I suggest checking this.
  Immersion: ★★
  
  
• Creatures respawn if killed by player: Allows every creature to respawn. By default big creatures and leviathans never respawn if killed by player.
  I suggest Leviathans_only.
  
  
• Leviathan respawn time: Time in days it takes a leviathan to respawn after it was killed.
  I suggest 3 days.

Lets briefly talk about some optional rules that can help you achieve extra Horror and/or Difficulty.

• No Exploiting Glitches
  This one is simple enough, no disappearing wrecks with the habitat builder, no flying/catapulting bugs, no wall clipping, etc etc.
  Difficulty: ★★
  
  
• 1Life
  1Life is easily the BEST way to make the game scarier. Adding stakes to dying will introduce true terror into your soul. One suggestion though, select Survival mode in the menu but delete your save if you die instead of selecting Hardcore mode. The reason for this is because selecting Hardcore mode will prevent you from saving via the pause menu, why does this matter? Because Subnautica crashes... A lot. Also, just because we are doing Survival doesn't mean Save Scumming is allowed. No saving before entering a dangerous location and reloading if it gets spicy.
  Horror: ★★★★★★★★★★

Thanks for reading!!!

Have an absolutely fantastic day.