Factual error: When Kong breaks free in New York, he stomps on people, but not an inch of blood appears.
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.
Continuity mistake: When Kong meets Dwan, he approaches his hand facing downwards. A shot later, it's sideways.
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: 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 wakes up, sits up on the bed with her hair all messed up and asks who picked her up from the dingy. A shot later, her hair is nicely brushed.
Continuity mistake: When the captain walks over to the bed where Dwan lies unconscious to feel her pulse, her head swaps positions between one frame and another and so do her legs.
Continuity mistake: Dwan's scarf falls through the side of the hatchway, but from the opposite angle, it falls through the middle.
Continuity mistake: When Wilson talks about a Beauty and the Beast musical, the light swaps from sunny to cloudy, and the wind swaps from mild to strong to breeze, all in a matter of sudden cuts.
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.
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.
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.
Continuity mistake: When Kong breaks the cage, the steps are packed with people. A mere second later, there's just Dwan.
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.
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.
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.