The Bill Gates Foundation recently sponsored a contest to invent practical toilets for the developing world. The winner was featured on NPR's Science Friday show. It's solar
powered and totally disinfects waste in 4 hours, (versus the extended time it takes a composting toilet to do the same) plus free hydrogen! They didn't give enough information to indicate whether the system could be used on boats, but perhaps it's a possibility?
Caltech toilet is No. 1 in Gates Foundation contest | Local News | The Seattle Times
Solar Toilet Disinfects Waste, Makes Hydrogen Fuel