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Other mistake: When Kong breaks free from the cage and the crowd runs away in panic before being stomped to death, there's an extra with white hair who is having the time of his life and can't stop smiling.
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Continuity mistake: When Kong breaks the cage, the steps are packed with people. A mere second later, there's just Dwan.
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.