@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:
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.