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: At the beginning when Dwan is tied to the podium and waiting for Kong she looks up and the trees are moving with Kong's presence. Then we see Kong approaching through the woods from a distance.
Continuity mistake: At the top of the twin towers, Kong hits the same chopper twice.
Continuity mistake: When Jeff Bridges and the girl escape while Kong is fighting the snake, they jump into the water and get soaked but a few moments later their clothes and hair are completely dry.
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.