heres how you do it
http://media.putfile.com/Under-the-Map-on-Crackdown
Tutorial: Once you exit the Agency tunnel go to the side and jump off the edge. You want to get high enough so that when you go into the water you can swim and get pushed under the side of the platform.
Face towards the water and jump and throw a grenade at the same time so that the explosion pushes you backwards. Ultimately this will push you far enough to where you can swim around under the map.
Tips:
If you don't get pushed under the platform, get out of the water and try again.
If you run out of grenades while under the platform and stil laren't free, have a teammate try to use their grenades to push you backwards. Otherwise, "EXTRACT" and try again.
Be careful of the sand and other low ceilings/barriers as you can get stuck