After receiving some armor, keep heading along until you walk up a fallen girder and pick up some ammo on the ground. Look down to your left and there will be a guard or two to take out in this room. Drop down to do so, find the health packs behind the crates. Pick up the heavy shotgun, take a step back, and blow up the containers to open the floor. You may have to shoot the floor one more time. Drop down into the ground and head for the waterway again straight ahead. Look familiar?