I think the drain plug was open in the AAV.
Actually, looking into this, there was a loose oil plug, which cause the transmission to fail, which reduced power in the engine - then there wasn’t ehpough power to run the bilge pumps at full speed, and so the water built up in thr compartment. The commander of the AAV should have started evacuation then, but instead flagged for help. A wave flooded the sinking ship, and the marines and sailor trapped inside wearing full combat gear couldn’t get out while the AAV tipped up and sank.