That's called the "Well Deck". I believe that is an LCM, Landing Craft-Medium, used to carry trucks, troops, & supplies to the beach, that is entering the Well Deck in that photo.
During a joint training exercise in South Korea, my USMC company was scheduled to re-embark on our ship via the smaller "P" boats, Landing Craft-Personnel (IIRC). However, the sea state was high enough that there were waves breaking INSIDE the Well Deck. We ended up flying out to the ship on CH-46 helos.
