Ok if toilet is manual, bad idea if toilet is macerating electric
Most raw water macerating electric toilets have integral intake pumps...iow, the intake pump runs when the head is flushed whether the seacock is open or not. Running it dry, with no water coming in not only fries the intake impeller, but also damages the intake pump housing. The toilet will not work when the seacock is opened, but will allow the bowl to flood with water and even overflow if the bowl is below the waterline.Intake seawater odor is a result of sea water left standing in the intake hose to stagnate while you're away from the boat. There's a simple cure for it that doesn't require messing with cups or a shower head: tee the head sink drain into the head intake hose. Flush normally with sea water until ready to close up the boat, then close the head intake seacock and fill the sink with clean fresh water...flush the toilet. Because the intake seacock is closed, the toilet will pull the water out of the sink, rinsing all the seawater out of the whole system--intake hose, channel in the rim of the bowl AND the head discharge hose.It is necessary to keep the sink plugged except when it's in use...otherwise the toilet will pull air through the sink and can't prime.