Both the head sink and the black water drain below the waterline.
Oh good... Here's my idea for you:
If the head sink drain thru-hull is below the waterline, theres certainly no reason why the sink and the toilet intake cant share the same through-hull, provided the toilet and sink are on the same side of the boat.
And its a simple matter to tee or wye the the head intake hose into the sink drain hose next to the seacock, 'cuz on most boats, both hoses are typically the same size: ¾ ID. Why would you want to do that? 'Cuz it gives you a means to flush the sea water out of the entire sanitation system--intake line, pump, channel in the rim of the bow, and head discharge line--safely. After youve closed the head intake seacock in preparation to close up the boat, fill the sink with fresh clean water and flush the toilet. Because the seacock is closed, the toilet will draw the water out of the sink, rinsing the sea water out of the entire system. If your toilet is electric, be careful not to let it run drydoing so can burn out the intake impeller.
It will also be necessary to keep the sink plugged except when in use, either by installing a conveniently located shut-off valve in the drain hose or with a rubber sink plug. Otherwise the toilet will pull air through the sink, preventing it from priming.
Do this any time the boat will sit, or as often as you think may be necessary to deal with head intake sea water odors.