All good points from @Wraith11B here. I would add the following to that:
-Capital ships take a long time to build, so start building early enough. If you're a major naval power going for BC, BB, and/of CV, you want to start building one or two units at the very start of the game (the 2nd one offset by 4-6 months) to get the practical up for your second run of more advanced ships, which should then be ready before the war starts. You can start building the more advanced ships before the first ones are completed without losing any efficiency in production.
-Escorts take less time to build, so you could delay building them until 1-2 years before the war.
-Major Naval powers don't start with a blank slate and it will cost significantly more leadership to go for a new path in doctrine and technology. For example, Italy starts with great BB tech, allowing you to start building the Littorio-class immediately. If you want to build CV's instead, you're going to have to start by researching CAG, then CVL, and finally CV, just to have basic CV's which are a lot lower tech than the BB's you can build at the start of the game.
- If you start with a pre-existing fleet you should research AA and doctrines for all the ship-types you have to get the most out of it . (unless you only have a couple of them and don't plan on building more, in which case you can relegate them to secondary roles, like shore bombardment.) This also means that it's more efficient to continue building the same types you already have in your fleet than to add new ship-types.
- Synergies in doctrines (CV-CL, BB-DD, and CA-BC-SS) and synergies in practical (BB-BC, CA-CL) should be kept in mind as well. Especially if Leadership (doctrines), or production capacity (practical) is in short supply. Combinations of both can also provide more versatile fleets: Cruiser Fleet (BC-CA-CL), Carrier-Cruiser Fleet (CV-CA-CL), Augmented Battleship fleet (BB- CA/CL/CVL - DD), Heavy Carrier Fleet (CV - BC - CA -CL)
- researching basic CVL isn't that expensive, and if you're going for a Battleship strategy and unless you mostly operate within interceptor range of your enemy, a single CAG can be very effective at protecting incoming naval strikes. Of course it will take significant damage, but it will greatly reduce damage to your ships.
- For CAG's in general, if you have CV's and/or CVL's, build extra CAG's and keep a close eye on their strength so you can swap them out for fresh units before they get destroyed. Basically, your CAG's will keep defending your ships, and attacking enemy ships in naval battles until they get to 0 strength and simply disappear.
- You should keep the speed and range of units in mind. You want your Carrier Fleets to have a high average speed, so adding a single BB to a CV fleet will likely bring down both the range and speed of the fleet, which could spell doom for the BB if you encounter a fleet with more/more modern BB's. If you want to make a combined fleet, your best bet is to do like the Japanese and operate BC's alongside CV's, or you can combine early game carriers with late-game battleships which have the same speed.
-SHBB is cheap to research and expensive to build. Can be built by the start of the war, and will win 1-1 from contemporary BB's, but by 1945 it will be superseded by regular BB's. They do have the highest shore bombardment value, so even when superseded, they remain better than BB's at supporting land forces.
-BC is faster and longer range than a BB, it's also cheaper, but it will loose against a BB. There are only two reasons to go for BC's: You want a big gun ship, but you can't afford to research/build a BB. You want a big-gun ship that can keep up with your CV's in both range and speed to create combined fleets.
- Escort ratio is a hotly debated topic, but what is clear is that you need more than 1 escort unit per capital unit in your fleet, if you're somewhere in the 1.5-2.5 escorts / capital ship range you'll probably be fine. CA counts as a capital ship.
- The AI isn't very good at the Naval game, meaning that even strategies that don't follow the most efficient/effective path can still work, as long as you have large enough fleets with a good escort/capital ratio and the AI can't consistently bomb the ports your ships are docked in. (whether that is because the enemy bomber's can't reach your ports, or because you have modern interceptors in sufficient numbers to intercept them.)
- Submarines are a separate game to the surface action, except for the fact that you need DD's to hunt them.
