Below The Waterline
It is possible to have a moon pool below the waterline and to keep water out of the chamber above it, if the chamber is airtight rather than open to the atmosphere above in any way. This arrangement is shown in part C of the diagram. Air pressure inside the chamber prevents water rising in the moon pool up to sea level. To keep the chamber airtight, access from the chamber to the rest of the ship is via an airlock with airtight doors. The design of the ship and its safety systems need to take into account the possibility of an air leak or catastrophic failure of the airlock.
In this arrangement the sides of the moon pool can be fairly shallow, and it can be used in a deep-draft ship without wasting space.
Read more about this topic: Moon Pool, Types of Moon Pools and Associated Structures, Above The Waterline