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: 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.
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.