Is there a through hull for a sink drain? Sink water can drain overboard without a problem and this is the normal setup on boats. To my knowledge no boats come with a grey water holding tank, just rvs. A small tank can be installed for your water supply or you could plumb to a portable tank in cockpit locker or wherever it can be placed. You will need a vent on the tank whether portable or not. A hand pump is easy to fit beside the sink. Hope this helps.