Hello, Kitty, and welcome to GameBoomers!
This is a tricky one for sure! We did have some discussion on it a while back, but I guess it must have scrolled off the board, because I couldn't find it. Anyway, the trick is to come up for air after you open the passageway, and then go down and swim toward a black area. Here is a quote from a different walkthrough, maybe it will give you some ideas: SPOILER
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Turn the sluice wheel to the right on entering. The water begins to rise.
Jump into the water.
When in the water, dive.
Underwater, down to the left, activate a second sluice gate hidden behind some detritus.
The water level rises even further.
Mount to the surface to take a breath of air.
Dive underwater into the small passageway which has now been made accessible.
Come out again into the sewers, on the asylum side.