@Moller A submarine is a sea unit with an attack value and a defense value. What makes it special are its “Special Abilities”/unit characteristics as described by the rulebook(s) and (partly) outlined by @WindowWasher above.
Those are:

Surprise Strike (X) Submersible (X) On Station Treat Hostile Sea Zones as Friendly (X) Doesn’t Block Enemy Movement Can’t Hit Air Units Can’t Be Hit by Air Units (X)

An enemy destroyer cancels those unit characteristics marked with an (X).

@Moller said in Battleship/cruiser with destroyer vs submarine:

I tought without a destroyer you can’t detect a submarine so you can’t attacked the submarine.

This is not part of the rules, sorry.