Revealing mistake: When Jack starts walking on the log, there are square chalk T-marks painted on it to point out where he must step, which disappear in the overhead shot.
Continuity mistake: When the three soldiers step off the elevator to fight Kong, the one on the far right bends down to pick something up. An instant cut later, he is standing tall next to the other two.
Revealing mistake: When Kong arrives at the top of the tower, his foot clips through the railing footage.
Revealing mistake: The flamethrower scorches Kong, who is behind an antenna. However, the antenna remains perfect, revealing it is a projection.
Continuity mistake: Inside the cargo hold, Kong throws a pipe on the floor. In the aerial angle, it's gone.
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 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 Dwan lies down and asks if she's going to Singapur, her left arm jumps from resting on her belly to across her body and next to the other arm, between shots. (00:20:37)
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 Dwan wakes up, she moves her left arm around her hair and nape. A shot later, her arm is lowered in a totally different position. Then, in an instant cut later, it's back around her nape. (00:17:43)
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 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: 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.
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.