Sea water or fresh makes no difference in the tank
There's so much bacteria in waste that the tank doesn't even notice a little more in sea water. Where it does matter is in the hoses and the toilet pump. Fresh water poured into the bowl only rinses out the head discharge line...it never gets into the pump or the head intake line. Teeing the head intake line into the sink drain provides a safe source of fresh water to rinse out the whole system. When closing up the boat, after you've closed the sink drain seacock, fill the sink with water...flush the toilet. Because the seacock is closed, the toilet will pull the water from the sink, rinsing the seawater out of the intake line, pump, channel in the rim of the bowl AND the discharge line.You can use the sink to flush the toilet all the time, but unless your boat is in especially foul water, that's more trouble than it's worth...'cuz you accomplish just as much if you only flush all the sea water out before the boat sits so it can't sit and stagnate...and stink.