Originally Posted by sailingdog
A better idea may be to run the discharge hose from the shower sump in the the same through-hull as the head sink. Grey water discharge is generally allowed in most places, so there is no need to dispose of it into the head and holding tank.
While it would be quite simple to plumb the discharge into the head sink outlet we simply have a hose that we drop into the sink itself when we've finished showering. I'd worry a bit about siphoning back into shower stall when on a (in our case port tack). I'd want to install a Y valve.