I do hope you mean the sink
drain under the sink??? /QUOTE]
You'd be right to question connecting any toilet designed to use SEA water to the fresh water plumbing, but apparently you've both managed to stay blissfully un-aware that there have been toilets designed to SAFELY use pressurized flush water from the boat's fresh water supply since the mid '90s (longer than that, actually...the VacuFlush was introduced in 1978...it never had any competition till the '90s). The toilet he's installing--the Raritan Marine Elegance, which has been the top rated macerating toilet since Raritan introduced it about 10 years ago --is one of 'em.
Raritan Marine_Elegance Promo.pdf Jabsco also has a couple of models designed to used pressurized fresh water. And there's one--the ONLY one--MANUAL toilet designed to use pressurized fresh water: the Raritan Fresh Head
Raritan FRESH_HEAD Promo Sheet
--Peggie