Red Pass Card in hand, drop in the water, pull the striped lever, grab a lungful of air, and swim down the passageway. When you make your first right, stop. Swim down the hole and go up into the area behind the turbine. Retrace your path to get some much-needed air*. Then head for the jail room platform - and the Yellow Pass Card door.

* There is a striped lever just to the left of the turbine in the main water room. This switch opens a door to the right of the turbine, which in turn makes for a very easy and fast escape once you've got the dragon. Stick very close to the walls if you go this route, the suction in the open water near the turbine is deadly.