Continuity mistake: When Kong swings the log, the man in the middle falls down and holds onto a branch. Barely a second later, he is standing up again and repeating the previous fall.
Continuity mistake: Kong throws a barrel at the soldiers, and Jack shouts, "Yes!" Then he rests his left hand on the wall. From the immediate opposite angle, his hand is now fisted. The hand's position is also inconsistent in the following shots.
Continuity mistake: When Wilson falls inside Kong's footprint, his scarf swaps between tied around his neck and loose back and forth between shots.
Continuity mistake: Inside the cargo hold, Kong throws a pipe on the floor. In the aerial angle, it's gone.
Continuity mistake: Dwan asks Kong, "I bet you are an Aries, aren't you?" with both of her hands lowered. A shot later, her left hand is on her head. (01:02:48)
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: On the boat, Wilson says, "He dresses like an ape and gets laid," then extends his hands and arms. From the opposite angle, they're not. (00:41:01)
Continuity mistake: When Kong is lying in the hold of the Tanker, Jessica Lange's character is leaning over looking at him when her scarf falls off. When it lands on Kongs chest, it appears to be the size of a (very) large dinner table cloth.
Continuity mistake: When the group of natives has discovered the Petrox crew and begins approaching them, a closeup shows Dwan moving from a crouching to a standing position and nervously taking a few steps back to stand behind Jack. The camera then switches to a long shot and she again stands up and moves behind Jack.
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: 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: On the boat back to New York, Wilson asks, "Any chance that you two are getting married?" and then Dwan's arms completely change position between shots. (01:29:07)
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 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.
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.