Naval warfare is about composition and the roles of every ship. Each ship has their own individual role. If one ship is missing or isn't doing their role right, you may have some problems in the naval warfare.

Capital ships: The main ships of your fleet, being big and expensive. These take longer to build than screen ships but compensate for that by providing a better armament, taking a heftier beating compared to screens, and, in the case of carriers, allow you to dominate the skies while at sea.

Screen ships: These are generally used to protect the larger, more vulnerable, capital ships that seem to not be able to handle a swarm of destroyers, light cruisers, and/or naval bombers.

Convoy: A "civilian" ship that has no defensive/offensive armament whatsoever. It's a weak and easy target for any ship. It is capable of trade, troop transport, and naval invasions.

Destroyer (abbreviated as DD): It is the second smallest ship (next to convoys) and is classified as a screen ship. Though it has the weakest naval armament in the game (excluding convoys), it is capable of delivering AA (anti-air) support to the fleet and has a (rather Godlike) ASW (anti-submarine warfare) weapon. It is effective in escorting convoys, taking shots from huge ships, and taking down submarines.

Light cruiser (CL): The third smallest ship in the game and is also classified as screen ship. It has a weak to moderate naval armament and has good AA support (although, it has minimal ASW weaponry). It is effective against destroyers, combating other light cruisers, and absorbing shots from other ships (most notably DDs).

Heavy cruiser (CA): It outguns the light cruiser and the destroyer and is considered a "capital" ship. It's a good opponent but easily sunk if not covered by other ships. It has no ASW but has a substantial AA armament. It's especially effective when combating screen ships such as the destroyer and light cruiser. It is easy to take out using naval bombers and submarines.

Battlecruiser (BC): This ship is basically a battleship that sacrifices a considerable amount of armor for speed. Though, it is an honorable capital ship that can take a lot of hits before going down, even with naval bombers. Its armament outguns that of the CAs, CLs, and DDs but it is still vulnerable to submarines if not supported by ASW capable ships. It's effective at combating screen ships, CAs, and taking down enemy naval bombers or CAS.

Battleship (BB): Never in any circumstance should you underestimate this capital ship. It is the iconic monster of the seas that needs to be respected. If in combat with one, make sure you have air superiority and naval bombers in that area of battle. Its armament can kill nearly anything that is weaker than itself, has an AA armament, but has no ASW weaponry. Therefore, if on its own, it is weak to submarines.

Super heavy battleship (SHBB): The Super heavy battleship is a substantially upgraded battleship that excels in pretty much every aspect except speed. That includes cost and manpower requirements. But coming to statistics, in-game, it's a giant ship (obviously) that can fight off anything that comes in contact with it. Although, it's weak to planes and submarines and has the same problems as a normal BB but amplified. If you field one, make sure to support it well. I promise you its worth it! Or perhaps others will just naval bomb it into the ground. Either or.

Carrier (CVs): These ships, as of right now, are (in my opinion) super broken as they can take down most of the enemy's fleet (using their naval bombers) without your ships reaching a single DD. While effective against any other ship, it has the cost of needing to be resupplied constantly with carrier-capable fighters and naval bombers. Generally, these ships are very good, when well supported, and are only countered if the enemy has air superiority where your carrier is. They are also one of the fastest capital ships. TIP: don't use more than 4 carriers in a battle as they get an air traffic debuff.

Submarines (SS): Warning! They are not underwater BBs! It's a good invention that can go underwater and shoot torpedoes at something. It is weak to anything that has ASW weaponry, specifically, DDsIt's basically useless in a combat role as the enemy will pretty much always have DDs protecting their capital ships. It's good if you need to disrupt a country's trade, as it does this by attacking enemy convoys. Since they have the lowest amount of HP in the game, it is generally advised to just go convoy hunting with them and hope for the best. Their armament consists of torpedos and minimal AA weaponry.

Fleet in being: A doctrine outdated in real life, but well-rounded in game. It relies on capital ships and destroyers to search and destroy the enemy fleet. It doesn't specialize in a certain playing style as much as the other 2 doctrines, but there is an emphasis on a capable surface fleet. This doctrine is the most widely used in the game due to versatility.

Trade interdiction: Gives bonuses similar to the fleet in being except that there is a huge bonus to submarines and a not as huge bonus to destroyers. It emphasizes on both submarine convoy raiding, surface convoy raiding. It also emphasizes on stealth in order to escape or bypass the enemy. If you are the weaker of the naval powers (or do not have a strong naval ally), I would suggest this doctrine as well as making subs and naval bombers.

Base strike: The carrier-based doctrine. It has an emphasis on carriers (obviously) and screen ships to defend the carrier. Great bonuses for carriers, screens, and subs. Limited bonuses for BBs and no bonuses for other capital ships. Would only run this doctrine if I know I'll be producing carriers and carrier capable planes, or better yet, if I have carriers to begin with.