Higgs already said it...
Composters and dessicators (the Airhead is a dessicator, not a true composter) do a fine job of drying out solids, but have two major drawbacks...they require power 24/7 to keep the fan and heater working--not very practical for a boat that doesn't stay connected to shore power...and liquids must held and disposed of ashore (except at sea beyond the "3 mile limit). If liquids must be held in jugs that must be carried ashore or in a tank that must be pumped out, I don't see much advantage to 'em over a portapotty or a traditional holding tank. The discharge of treated waste from a CG Certified Type I MSD is legal in all FL waters except the Keys...so for about the same price as the Airhead, Bob could add a Raritan PuraSan http://www.raritaneng.com/products/waste_treatment/purasan.html a CG certified Type I MSD that treats a flush at a time and discharges it overboard with -0- harm to the environment, and needs no power except during the 2 minutes it's actually running after each flush. Why store waste aboard if you don't have to and can discharge it legally and without harm?