Instead of creating the usual steel turnstile, the Watergate's designers used the primordial liquid as a psychological barrier. Their logic: People won't like to get their clothes wet. Obviously, they don't live in NY summer and they wear pants.
Or in other words: If they install this on the NYC subway, I would not spend a single cent on my Metrocard. It's a good idea, because most people will actually respect it. Another good thing: If something happens, people can run to the exit without having to go through gates:
Water is only a psychological barrier. Fleeing, panicking persons can escape through the gate without being hindered by any rigid media.