Instead of converting to any marine toilet (all of the Raritan models #tfox mentioned are high end macerating electric toilets that would be waaaay overkill for a 25' boat!) which will also require adding a holding tank and all the related plumbing needed with them that may include installing a new below-waterline thru-hull for the flush water intake line if there isn't one already installed, there's a Plan B makes a lot of sense on a boat the size of yours: An "MSD" 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 including urine jugs 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--and also a deck pumpout fitting if you don't already have one installed is about $250--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 AND a size and height that isn't like squatting on a bucket!
If this idea appeals to you check out the Dometic (formerly SeaLand) 975 MSD SaniPottie.
A number of people here who have 25-29' have installed MSD portapotties and love 'em...hopefully at least a couple of 'em will chime in here.
--Peggie