If you plan to continue only day sailing once a year, the Johnson AquaT is an ok choice. However, that means your boat and everything on it will spend a lot more time sitting than being used...and lack of use is harder on all equipment than hard use because --along with everything else, lubrication settles or dries out, ceasing to provide any protection from corrosion and dried out rubber parts.
You'll also have to use a holding tank, which means replacing all the plumbing to and from it, so I have what I think may be a better idea: replace the toilet and the tank with an "MSD" portapotty."
On any boat much smaller than about 30' a self-contained system—an "MSD" portapotty--makes a lot of sense. The "MSD" designation in the model name/number means it has fittings for a pumpout line and vent line, and is designed to be permanently installed (actually just sturdier brackets than portables, so you could still take it off the boat if you absolutely have to), which means that although it's still called a PORTApotty, you don't have to carry anything off the boat to empty it.
A 5-6 gallon model is household height and holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet. No plumbing needed except a vent line and pumpout hose--so no new holes in the boat...and -0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $300--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space. The Dometic Sanipottie 975 is the 5 gal model and is household height Defender has it with MSD fittings for $299... the 974 is the shorter 2.5 gal model for $229 at Defender.
--Peggie