'Cuz it can't be done without risk of contaminating the potable water supply or damage to the toilet or both....And every toilet mfr specifically warns against it in their installation instructions. Some people may argue that you can protect your fresh water from contamination with a system of siphon breaks and one way valves...but protecting the pump from damage isn't that easy, 'cuz sea water pumps are designed to PULL water through 'em. Water in the fresh water system is PUSHED through the pump, which can play havoc with the rubber parts in a manual toilet. Toilets that are designed to use pressurized flush water have a solenoid valve that the flush button opens and closes, which keeps the pressure off the pump.

There's only one reason to use fresh water: eliminate sea water odor issues. And there is a very simple safe way to accomplish that: Tee the head intake line into the head sink drain line close to the the thru-hull (this also has the advantage of eliminating one hole in the boat or freeing up a thru-hull for use by something else like a washdown). To flush using sea water, just keep a plug in the sink. To flush using fresh water, close the thru-hull...fill the sink with fresh water. The toilet will pull the water out of the sink, rinsing the sea water out of the ENTIRE system--intake line, pump, and channel in the rim of the bowl in addition to the discharge line. Sea water in open ocean is actually pretty clean...it's coastal water that can be really skanky. This setup will allow you to conserve fresh water while at sea...use fresh water all the time while inshore. Simple, inexpensive and safe.