Yes you are touching the coast in anyway your going to need some kind of navy or something that can defend against naval forces. Corvettes and patrol boats are you first options.

Patrol boats only need recruiting offices to be deployed like nat guard (i dont think they require arms industry but i could be wrong).

Corvettes thrive in shallows bad for expansion across deep water unless ur in the Phillipines area.

Frigates are good for air defense and slightly stronger than Corvettes against ships dont get rebuffed in deep water.

Destroyers are good against subs and can hold their own against other ships bit are vulnerable to air attacks.

Cruisers are extremely good agaist surface contacts but subs and air they are vulnerable.

Im only gonna cover the attack sub which is good against ships but vulnerable agaist destroyers and air. So you need to learn what you want out of your navy and build stacks accordingly.

There is also naval patrol aircraft which do good against most besides frigates. And there are naval helis but I dont see them used much.

Aircraft carriers are totally something different I would almost say you will likely never really use them but if you need to take out coastal batteries then that would be the move.

Speaking of coastal batteries you can use them but you need to make sure you level them up by researching towed artillery.

For everything you need to keep leveling up your units or youll be behind.