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.
Continuity mistake: When Kong breaks the cage in New York, the human-in-disguise gorilla is different from the mechanical model used in the wide shots, most noticeably the mechanical stiff movements and the dirty teeth that swap to white and pearly.
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.