There's a very simple alternative
That will eliminate the need to use any chemical to eliminate sea water intake odor: tee your head intake line into your head sink drain line. When closing up the boat, after you've closed the seacocks, fill the sink with clean fresh water and flush the toilet...because the seacock is closed the toilet will pull the water out of the sink, rinsing the sea water out of the entire system--intake line, toilet pump, channel in the rim of the bowl AND the head discharge line. Follow that with a cupful of white vinegar--which should be flushed completely out of the pump--to eliminate sea water mineral buildup. Total cost of materials: less than $10...no more cartridges to buy.