A common problem with sea water toilets
Sea water is alive with micro-organisms. When the boat sits for a few days they die, decay in the head intake line, pump, and channel in the rim of the bowl...so the first flush when you come back is VERY stinky till all the stagnant water is flushed out of the system.There are inline chemical devices that can be installed in the head intake line which are supposed to eliminate this odor...but I don't recommend 'em. They're expensive, often messy...their effectiveness varies...and the chemical can react negatively with holding tank products. The most effective least expensive way to prevent it is to install a way to purge all the sea water out of the system. Just pouring clean water into the bowl and flushing won't rinse out the intake because nothing poured into the bowl is recirculated through the intake. My favorite solution is simply to reroute the head intake line...tee it into the head sink drain line, so that it shares the same through-hull. At the end of the weekend, after you've closed all the seacocks, fill the sink with clean fresh water...flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink...flushing all the sea water out of the whole system--intake AND discharge line.If that's not possible, another solution is to tee a short a short piece of line--using a y-valve--into the head intake line that you can stick into a bucket or gallon jug of clean fresh water.