@Moller
No Destroyer Present: A sub cannot be hit by air, can sneak attack, and can submerge whenever it wants too
EX: A sub is attacked by a battleship, a cruiser and a carrier with 2 fighters. The submarine can choose whether to fight or not because there is not destroyer present to force it to fight.
Destroyer Present: A sub can be hit by air, cannot sneak attack, and cannot submerge.
EX: a Sub is attacked by a Battleship, Cruiser, destroyer, and a carrier with 2 fighters. The sub cannot submerge and must fight. The sub also can be hit by the air units on the carrier, but it cannot ever hit them back.