A method and apparatus for cargo transfer at sea, more particularly, a flow-through vessel and related method for the at-sea and underway launching and loading of air-cushion vehicles. The vessel includes a hull having a forward end and an aft end, a continuous deck extending from the forward end to the aft end of the hull, a forward ramp attached to the continuous deck at the forward end of the hull, and an aft ramp attached to the continuous deck at the aft end of the hull. At least a portion of the continuous deck is uncovered and accessible from above the deck.