Check any sink with a magnet. The magnetic ones rust like hell in the tropics. Ditto stainless galley ware. Some run the drain pipe under a standard marine head seat. Others simply dump the contents in the head .Both options eliminate the sink drain thru hull.
Bedding it down in Urethane eliminates the need for tie downs.
Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"