There is a joke about a 'poop deck' in here somewhere....
Before this goes off in the wrong direction, are you asking about a self contained unit, like the Chemical/RV toilets; or are you speaking of a biodynamic composter? If the latter, are you going to garden on the barge? Where will the compost be used in your floating paradise?
Nature's Head is a brand of composting toilet designed for sailboats. It needs a little power to run a tiny fan. Works well for two. Cheaper than most commercial composting toilets, but still not cheap.
Lovable loo sawdust buckets are the most compact diy version I can think of.
Bigger question is what do you do with the doo every week or two? Does it have to stay on the barge or do you have a place to compost on land?
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
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"Family farms work when the whole family works the farm." -- Adam Klaus