Revealing mistake: When Kong picks up the subway car packed with people, in the wide shots, one can see it's empty.
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.
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.
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: 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.
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 grabs Dwan, the bracelet around her arm disappears and reappears randomly.
Continuity mistake: When Kong throws metal objects at the soldiers, their shape differs between shots.
Continuity mistake: When Kong is shot, Jack leans both hands on the window. A shot later, they are lowered.
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.
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.
Continuity mistake: At the beginning when Dwan is tied to the podium and waiting for Kong she looks up and the trees are moving with Kong's presence. Then we see Kong approaching through the woods from a distance.
Continuity mistake: At the top of the twin towers, Kong hits the same chopper twice.
Continuity mistake: When Jeff Bridges and the girl escape while Kong is fighting the snake, they jump into the water and get soaked but a few moments later their clothes and hair are completely dry.
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.