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"if bacteria could swim backwards up the bowl, "
There's no "if", they can and do. In home toilets with a flushometer, there's a considerable pressure head in the system and an air bleed in the valve, so the standpipe isn't full of water after the flush--it's an air gap.
WIth toilet tanks, there's also an air gap, the fill pipe is set a good inch above the water level in the tank, among other precautions, so the incoming water is again isolated from the bowl. Even in sinks and tubs, there are requirements that the sink spiget is located above the level of the overflow drain, so there's always a break between the water source and the basin.
Considering that a marine head isn't set up with the necessary siphon design to "flush" by incoming water and has all those valves and things to much it up...Your simplest solution is probably to set up a day tank in the head with fresh water. Fill it from an overhead spigot (so there's that break again) and plumb that to flush the head with fresh water. Resist the temptation to use pressure water for the head--because that would also fill your holding tank in record time.
When shore water is hooked up to a boat for any reason you also have a risk of flooding and sinking, because that shore pressure is too high and if you put a regulator in line, sometimes they still fail. Better to run the shore supply to a tank, and use a ball valve in the tank inlet to securely close off the shore supply except when filling the tank--unless your whole system was designed for high pressure water.
In NYC last week, there was a water scare over perchlorethane (dry cleaning fluid) found in the domestic city water supply. It took a week but they traced it to a car wash with an illegal connection to the highly pressurized city water supply--which somehow "sucked in" the tainted water being dumped at the car wash, even against the high pressure supply.
Last edited by hellosailor; 05-13-2007 at 08:39 PM.