I maintain a powerful navy.
Basically, each nation can have 3 fleets.
"Fleet command" (best for strategic warfare)
"Helicopter Carrier" (Best for defensive)
and "Jet Carrier" (Best for offensive)
You need to have a decent level of supply ships, but not too many, and defensive destroyers in all. Helicopter carrier allows you to have naval helicopters, that are labeled as "offensive" because they can strike out at any enemy force. Do not use these to attack enemy cities, forts, and yadda yadda. Use them to attack enemy forces within' your own borders, and equip them with decent(crossed out) GOOD defensive naval weapons.
Fleet command is only good for strategic warefare and naval combat. Load with nuclear subs, defensive destroyers, submarines, and attack destroyers and boats. Use them in nuclear combat and attacking enemy fleets.
Jet carriers need defensive destroyers, around 400 decently armed jets at HIGH quality (buy at Q240+ because naval corporations need to hire at 10% and produce at 10% until they upgrade decently and order decent supplies), SEAL units, and attack destroyers. Use these fleets to kill off enemy jet fighters and interceptors, as well as saturate enemy targets and paint the coasts if existent.
Navies are insane cost and insane powerful if you know how to use them. They are rare and underused, so if you care to learn naval war strategies, contact me. If you have a pop below 30M-35M, have only one fleet, and choose wisely. At higher populaces, they are good if you know how to use them.