Gears of War 3 Game Guide

Act Four: Chapter 6 – Watery Grave

Just kill the monsters when they attack, shoot the mines in front of the sub,
shoot the big monster in its mouth, hit three mines while being swept away 
(they should all be in front of the sub) shoot torpedoes down, kill the
turrets, shoot the door, watch for incoming torpedoes again, and then kill the
final turrets to end the chapter.