Level Skip: 
While playing a one-player campaign, hold L1+L2+R1+R2+Select 
then press Start+X. 

To Kill a Battleship:
The easiest way to kill a battleship is with a submarine. As soon 
as the battle starts, DIVE! When you dive, a helicopter will come out 
and use sonar to find you. You should make your way under the other 
ship. While staying under him, pump out torpedoes like heres no 
tomorrow. In this way he should be destroyed fairly easily. 

Trash a Carrier with your PT Boat: 
When the battle starts, let him target you and launch his planes. Then, 
extend your hydrofoils and turn straight into him. His planes will miss 
you (most times), and won't be able to land when he's turning away 
from you. Keep turning with him as you light him up with torpedos.