Visible crew/equipment: When Jack walks into the hotel suite, a crouching boom mic operator and the mic pole he holds are reflected in the mirror.
Other mistake: During the last shot of Dead Kong, one of the extras in the crowd waves his arm at the camera.
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.
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.