If it's where I think you are, you have to go up the ladder on the side of the beam and jump onto the pipes ahnging from the ceiling. Follow them to get up to the ledge you couldn't get to by jumping. In the other room you will find a valve to turn that will bring up the water level. Go back to the other room again and go down the ladder this time. Now you will be able to swim through the big pipe and to the other side.
