The story
From moon + pool, from the fact that, on some nights, moonlight can be seen reflected by the surface of the sea through the opening, making it resemble a swimming pool.
Etymology from Wiktionary, CC BY-SA 4.0.
moon pool
Today it means: An opening in the base of the hull of a vessel (especially a marine drilling platform), which is used for lowering equipment into the sea below. Full definition →