Aloft—Up above.
Alongside—Close to the side.
Amidships—In line with the keel.
Anchor—The instrument used to hold a vessel to the bottom, usually made of iron.
Astern—Behind the vessel.
Athwart—Across, as athwartships, meaning that a thing is lying across the vessel.
Avast—An order to stop.
A-weather—An order to put the helm towards the windward side.
Ballast—Weight placed in or hung to the bottom of a boat to keep her upright.