Start off Mission 7. This will be much easier if you do it on Agent. As soon as you start off against the Brothers, go down one floor so you're on the left side of the boat. You'll see these columns that you can take cover behind. Go up to the second one and take a gun without a scope, (Magnums, CMP-150s and Uzis work best) and press LEFT-TRIGGER, A, A really fast behind cover.
Soon you will enter the wall, and you then press LEFT-TRIGGER and A and COMBAT ROLL into the wall. This will send you underwater, allowing you to se the Trinity Base from the outside. Underwater. With no air supply. And you don't die.UNLESS you go to far off the rock ledge you fall on, in which case you will fall out of the map to your death.