On a 25' boat--on any boat under 30', I'd go with an "MSD" version portapotty. 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 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 $200--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. For just a little more, you could add a y-valve and manual diaphragm pump to also give you ability to dump the tank (only where legal of course). 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.
Check out the Thetford 550P MSD and the Dometic/SeaLand 975MSD Sanipottie