Next step ain't easy. Take a deeeep breath and dive into the door opposite to where you found the hammer.
As far as I remember:
* immediately go right and swim to the end
* there, turn left and swim almost until the end
* just before the end, at the last crosspoint turn right and immediately left again. You should come to a small room with a ladder going up to a lock.
Use the hammer at the lock to break it, this will make it. Good luck!