Continuity mistake: When Kong is shot, Jack leans both hands on the window. A shot later, they are lowered.
Revealing mistake: When Kong is being burnt with the flamethrower, the flames have a clear clean cut on the left edge, revealing where the composite image was placed. In some shots, you can see the whole outline of the pasted image.
Revealing mistake: When Kong is atop the Twin Tower and throws objects to the soldiers below, check the last one: there's a blatant metal lid on the set ahead in the very same spot where an explosion occurs seconds later, most surely where the pyrotechnics were hidden.
Visible crew/equipment: As King Kong jumps from one of the Twin Towers to the other, you can see the wires attached to him as he lands. (01:59:30)
Answer: He probably knew it wasn't deep. It's his island; he knew every inch of it. Besides, it's a pool, not an ocean. It most likely had a small underground tunnel which spilled out somewhere on the island.