Naval unit guide
For the start of the game, Frigates are the most effective ship in the game. In large numbers (1000+) frigates will beat any late game enemy fleets. Though it may severely lag the game, and crash your computer if you try to look at any battle using 1000 frigates.
Commerce raiders are only better than frigates if you need a fast fleet. This is because for every one commerce raider you can have, you could instead have 3 frigates.
Large numbers of ironclads are needed if you want to colonize effectively, and also happen to be the counter to mass frigates in multiplayer (300 ironclads beats 900 frigates).
Cruisers are the best ship late game. This doesn't mean that you should spam 100% cruisers. Building huge cruiser-only fleets means that you will miss when trying to kill dreadnoughts with them. This means that you want to try and maintain a small lead in ship count, with around half of your supply limit being used on dreadnoughts.
Ships that never need to be built: Man o' war, Commerce Raider, Monitor
Naval blockades can provide up to 25 war score if prolonged and no other battles occur. They also increase the target nation's war exhaustion. They only harm the target's economy indirectly (War exhaustion makes factories less profitable).