What is your source of flush water?
Is it raw water, or are you pouring water down it to flush? The toilet isn't the problem...Foul odors for the first few flushes after the boat has sat are caused by water left to sit and stagnate in the head intake. Though the problem is more common in salt water, it can happen in fresh water that has a high sulphur content, polluted by runoff, or is highly "organic" (relatively shallow, lots of animal and vegetable sea life). So if you only have odor after the boat has sat, and if it only lasts for the first few flushes, the head intake line has to be the source of your odor...and you can't cure it by pouring anything into the bowl 'cuz nothing that goes into the bowl is recirculated through the intake. Even if you've tried to solve the problem by closing the head intake and pouring clean water into the bowl, until you clean out the head intake line the problem won't go away. You may have pulled in some animal or vegetable life that's died and decayed in the line. So the solution is: clean out the head intake line. Disconnect it from the thru-hull (close the seacock first!) and stick it in a bucket of water that's liberally laced with C.P. Flush it through the head. If you see a lot of black flecks in the bowl, you may have to flush several bucketfuls to completely flush out whatever's died and decayed in it. To confirm that you've solved the problem, reconnect the head intake line, but don't reopen the seacock...flush by pouring fresh water into the toilet. When you come back to the boat, you shouldn't have any odor when you flush the first time, also using water poured into the bowl. There is a permanent fix that will allow you to rinse out the head intake before closing up the boat. We'll get to that after you've confirmed that the head intake is the source of your problem.
