he he. are U talking about that helicopter thing where you have to shoot from the ship while it sinks.. ??
this is my method.
Shoot like 6 or 7 rockets directly before the ship sinks down. You can actually go to the rear part of the ship where you have extra rocket pod ammo. That part of the ship doesnt sink totally... You have to shoot like 10-15 rockets directly before its damaged & fly away.
2nd method is. Arm your self with rocket pods, then blow the ship. Now swim to the small island which you came from. There is a coconat tree that you can use to hide behind & if you're fully armed with rockets thats all you need to damage the heli
Hope this helps...