Continuity mistake: When Kong breaks the cage, the steps are packed with people. A mere second later, there's just Dwan.
Other mistake: When Kong breaks free from the cage and the crowd runs away in panic before being stomped to death, there's an extra with white hair who is having the time of his life and can't stop smiling.
Revealing mistake: When Kong pops out of the EL tracks and extends his hand, the glove is noticeably cracked, and the stuffing is coming out.
Revealing mistake: When Kong grabs the subway car and throws it away, the lower part of the car is a blatant miniature model with no cables and no wheels.
Revealing mistake: When Kong fights against the helicopters, the real helicopters have red and yellow lights all over, but the miniature ones have just a couple, revealing the FX trick.
Continuity mistake: When Kong grabs Dwan, the bracelet around her arm disappears and reappears randomly.
Continuity mistake: After Jack walks over the log and crosses the cliff, his rifle is strapped to his back while he turns around to watch Kong. A shot later, the rifle is already in his hands.
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.
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.