46 CFR §185.330

1. (a)Except when operating on lakes, bays, and sounds, or rivers routes in calm weather, all hatches and openings in the hull, except loading doors, of a vessel must be kept tightly closed except when being used.
2. (b)All watertight doors in subdivision bulkheads must be kept tightly closed during the navigation of the vessel except when being used for transit between compartments.