Revealing mistake: When Kong arrives at the top of the tower, his foot clips through the railing footage.
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 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: Inside the cargo hold, Kong throws a pipe on the floor. In the aerial angle, it's gone.
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.