Every Stormworks player has probably fallen off their boat once or twice - and the feeling when you watch your boat speeding away, knowing you cannot do anything to stop it, is terrible.
Thankfully, Stormworks has the possibility to stop a machine when a person is no longer operating it.

Picture you're seated on a big lawn mower, but you then fall off and are severely wounded by the spinning blades.
If that mower had a 'dead-man's switch', which would probably be in the form of an elastic band from the key in the ignition to a loop in your belt, then IF you fell off, then the key would be yanked from the ignition and the engine (and thus the blades) would stop, and you could get back on the stopped lawnmower at your leisure.

Dead-man's switches are especially useful with Stormworks, because if you cannot return the vehicle to the workbench in Survival mode (or just don't want to), then the boat won't ride off over the horizon the moment that you try to climb around it.

P.S.: MAJOR NOTE!!!
Throughout this guide I refer to a block called a 'Player Sensor' as a 'Player Detector'.
This makes no difference so I won't bother to change it, just... be aware. It's Sensor. Not Detector.

Usually, when creating a guide, I would include pictures - but as of Stormworks 1.0 release, dead man's switches have become ridiculously simple because the only building mode available is Advanced.

So! Let's get into it. BTW, I will refer to dead man's switches as DMS.

First, get 2 things into the building hotbar:
1.) A player detector
2.) A fuel valve

Now, find a good place to put the player detector. Since they only have a max range of 10 meters, which isn't even enough to cover a boat the size of the starter dock, I prefer to put it at the very rear of the cockpit. Or bridge. Or driver's seat. You'll see why soon.

Then, find the pipe down in the engine room through which the engine's fuel supply goes.
Delete 2 sections of this pipe and replace them with the fuel valve. Do this once for every separate fuel supply pipe.

Now, go into Logic mode and link up both components to a battery. This will ensure that they are powered.

Now, link the 'Player Detected' node on the Player Detector up to all fuel valves you have placed.
Go into 'Select' mode and put the 'Player Detection Radius' to the highest it will go.
Done!
No, seriously. That's it. Test it out - After starting the engine, at ANY speed if you go 10m away from the player detector at any direction the engines' fuel supply will be cut almost instantly and the boat will slow to a stop.

This is more of an upgrade of the Basic.
So, let's say you have a large boat and are playing alone.
You're on a long journey over the open ocean, or have an autopilot, or whatever. If your boat is longer than 10m, which it almost certainly is, then the moment you leave the cockpit without shutting off the engines the basic DMS will activate. This is kind of annoying; so, I've come up with a (hypothetical) solution.*

*I haven't tested this system yet. I will as soon as possible.

The most obvious thing to do would be to stick MORE player detectors* on a ship. But what's this? You can't put more than one logic link onto the valves?

*Henceforth to be referred to as 'PD's'.

Logic gates to the rescue!
Oh, please come back...
Logic isn't that scary...
I felt the same way.

But don't worry, I'll explain it!
So, all we'll need in this (very short) build is:

1.) ?x OR gate
That's it.

First what you want to do is find a nice interior place to fit a bunch of logic components. This system won't be complex, but it can take up a bit of space.
Now that you have a bit of space to work with, select your first OR gate.

OR gates are commonly referred to as the 'Any or all' gates. And it's exactly what we need.
Say you have 8 PD's which are hooked up into OR gates. If ANY of them or ALL of them detect player(s), then the OR gate will let a signal through.

If NONE of them detect a player, it means that player has probably slipped on a wet bit of deck and fallen overboard. The OR gate then won't output a signal because NONE of the PD's detect anyone.

What you will want to do is put down half the number of OR gates than your number of player detectors, rounded to the next even number; if you have 8 PD's, then you will put down 4 OR gates. If you have 7 PD's, then you will still put down 4 OR gates because each OR gate has 2 inputs.
Now, you will want to link up all of those PD's to all of the OR gates' input nodes. Then, count how many output nodes are on all those OR gates and place down half THAT number. Hook up all the OR gates' output nodes to the new OR gates' input nodes. Then, count the output nodes of all the new OR gates and place down as many OR gates as half of THAT number...

I think you get the point. Basically, we're trying to make an OR gate upside-down pyramid to reduce the number of output nodes to 1 - to finally be able to shut those fuel valves.
Once you've made your figurative upside-down pyramid of OR gates, and have just ONE output node left, you can link that output node to all the fuel valves that the Basic DMS you built before. If you add more PD's, then simply add more OR gates.

Now, test it! It should function the same as the Basic DMS, but with coverage over your entire boat.

I hope that this helps!
Happy sailing,
- Jeb T. Firefly