Continuity mistake: When Kong dries Dwan's hair by the waterfall, she is holding onto his second finger. From the opposite angle, she is holding onto his third finger. This swaps back and forth.
Continuity mistake: On the island, Jack removes the lens from his camera. A shot later, the lens is back on, and he removes it again.
Continuity mistake: On the island, when the group admires the huge picket fence, the man on the far right watches it with his binoculars. An instant cut later, his hand is lowered, and he is grabbing them again. This happens several times throughout the scene.
Continuity mistake: When Dwan's inside the cargo hold, her trousers are grey. When she's outside, they're blue.
Continuity mistake: When Kong breaks the cage, the steps are packed with people. A mere second later, there's just Dwan.
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.
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.
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.
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.