Originally Posted by sailingdog
The simplest way to do this is to eliminate the sink drain seacock and diverter valve at the sink. There's no reason you can't drain the sink through the head intake seacock. If you want to flush the sink using fresh water from the head, just close the seacock. This is actually the method suggested by Peggie Hall, diva of marine heads and plumbing.
I think you responded within about 90 seconds -- make that 60...
Your suggestion (and Peggie's) is even simpler, except that...
a) the head intake seacock is under the vberth and access requires either lifting cushions and an access panel or pulling out a drawer and reaching a long way (if it's even reachable). I've updated the image to indicate the forward bulkhead between the head sink and head intake. As it is in the diagram, I can leave the head intake seacock closed (so long I want to flush using fresh water). I routinely close all my seacocks (except cockpit drains) when we leave the boat.
b) Removing the sink drain seacock turns this into a much bigger project if I wanted to remove the thru-hull. I suppose I could just cap off the seacock... Ironically, I actually just replaced it last year -- completely seized (open!) and badly corroded.