What do you call the sections below the main deck that can be used for storage or living?

The term that refers to the sections below the main deck used for storage or living is "compartments." In naval terminology, compartments are designated spaces within a ship designed for specific functions. These can include living quarters, storage areas, engine rooms, and many other purposes. Each compartment is designed to optimize the use of space aboard the vessel, ensuring that essential needs are met, whether for crew living arrangements or for storing supplies.

The other options reference specific areas or concepts on a ship but do not encompass the broader use of space like compartments do. For instance, the galley specifically refers to the kitchen area on a ship, berthing pertains to sleeping quarters, and freeboard refers to the distance from the waterline to the upper deck of the ship, which is important for stability but does not imply any use of space for living or storage. Therefore, "compartments" is the most comprehensive term that accurately describes the areas below the main deck utilized for various functions on board a ship.