Moon pools are openings in the floor of a marine structure or vessel that allows direct access to the water. Even if the structure is entirely submerged, careful use of pressure can ensure that the water level does not rise above the floor level and the room is not flooded. Moon pools can be used to allow quick entry/exit of divers, equipment, and watercraft between the marine environment and a protected interior.

Moonpools may have cranes positioned nearby to aid in moving equipment in and out of the water.