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: 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: 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: 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: 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: 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 boat is arriving at the shore, the captain grabs his radio and speaks, then lowers it. A shot later from a different angle, it's close to his mouth again.
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.
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: 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 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.
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.