You're going the wrong way. Swim back into the big pool, and you'll see at the bottom of the pool a cage with barrels in it. Give the lock a whack with the crowbar, and, when they reach the surface, use them as a bridge and take a flying leap onto the real platform. Make your way as high as you can, and cross to the other side of the vast wall that splits the pit.
Now you'll have to go through the underwater tunnels until you find one that seems to be right!