A Special advanced tec defense made for anti-ship only

I think there should be a special defense for taking out ships like defenses that you are able to place in the water just incase your enemy spams water units

So, some Radar and a Catapult or two then?

Navy always has one major disadvantage; It can't leave the water. Your ground-based artillery far exceeds the Navy's range so Lobbers and Catapults (and even pelters) are your friends.

Then there's also bombers. Even T1 bombers make a mess of Navy, and they're by and large cheaper to produce too.