After opening the trapdoor in the mazelike area with the swinging chains, as shown in the previous section, jump into the water.
Roll and swim through the flooded passageway to the north (the only other way out of this room). Follow the passageway forward then left.
Swim straight across the large room with the square, metal pillars, through the gate on the west side. (This gate will be open if you pulled the chain in the ROOM WITH BRASS ARCHES AND PIPES after defeating the mechanical horseman.)
There are no health/ammo pickups in this underwater area, but if you like to explore, note that there is an air pocket in the ceiling of the large chamber just before the gate. (If you look up through the grating, you can see the pull chain in the room above.)
The side passage on the right (north) leads to the other trapdoor in the mazelike area with the swinging chains. The trapdoor only opens from above, but you can also get air there.
Behind the gate on the west side of the flooded area is a small room where you can climb out of the water and use your crowbar to pry 2 GOLDEN STARS from their receptacles.
When you have the two artifacts, swim straight across the large flooded room to the opening where you came from. Turn right...
...swim into the small room ahead and surface at the trapdoor.