Revealing mistake: When Kong climbs up the Twin Tower, Dwan is replaced with a blatant doll.
Revealing mistake: After Kong takes Dwan out of the water, she is replaced with an unrealistic doll.
Deliberate mistake: When the natives abduct Dwan, they grab her and leave in a nanosecond, leaving nothing on the deck. When Jack arrives, there's suddenly a necklace on the deck which helps him figure out what went on, despite being a ludicrous and deliberate McGuffin. Check the beer can and red box as a reference.
Continuity mistake: When Kong meets Dwan, he approaches his hand facing downwards. A shot later, it's sideways.
Continuity mistake: Before Dwan is abducted, the platform where she is fishing is fully lit and has objects scattered everywhere. After Jack leaves, the place is tidier and in the shadows.
Continuity mistake: When Jack is about to steal a boat, Dwan has a book lying on the right. A shot later the book has changed positions.
Continuity mistake: Dwan's scarf falls through the side of the hatchway, but from the opposite angle, it falls through the middle.
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)
Continuity mistake: When Dwan's inside the cargo hold, her trousers are grey. When she's outside, they're blue.
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: 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.
Revealing mistake: In the wide shots of Kong carrying Dwan around the barren lands, she is replaced by an obvious doll with stiff hair and unrealistic short arms.
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 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: 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.
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.