Composting can be an excellent alternative...
But as you saw on the Sun-Mar site (they make the best self-contained composting toilets, btw), they're HUGE--they need a space about 20" deep x 25" wide by 29" high--more than most heads have to offer...and they have to be. There have been some attempts at smaller units, but they don't hold enough to give the material time to develop into finished compost. The biggest drawback--other than size--to their use aboard is the need to deal with excess liquids...we urinate a LOT more often than we defecate, and wet soggy organic material doesn't compost, it rots. The units are equipped with evaporation chambers that consist of a warming plate and fan, but often there are more liquids than it can handle...and they can't be legally drained overboard (whether urine is or isn't sterile is irrelevant...the law doesn't distinguish between liquid and solid waste), so they have to go into a holding tank. So there goes any advantage. There are other issues...a 3" vent stack is required...it's necessary to add a cup of peat moss to every deposit (which means carrying a supply aboard. However, the cost (about $1200) is about equal to that of an electric macerating toilet and a holding tank...and less than the combined total of the same toilet and treatment device. Bottom line: composters are great in cottages where there are no sewers and septic tanks cannot be installed...I've seen 'em used very successfully on some houseboats...but they're a wee bit impractical on a sailboat.
