(Note to anyone else about to chime in: he's in Greece!)
A simple manual toilet is a reasonable solution for a 26' boat ONLY if you don't also have to install a holding tank too...required in some areas of the Med and Agean, but not all...so it depends on your "cruising grounds." If you do, you'll lose at least 25% of your storage space to the tank and related plumbing and move from a no-maintenance toilet to a high maintenance sanitation system.
But you don't have to settle for a chemical toilet (portapotty) that you have to carry off the boat to empty it. 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 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. 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. In fact, you can even add a y-valve and pump to the pumpout line to let you dump the tank where legal.
Check out the Thetford 550P MSD and the Dometic/SeaLand 975MSD Sanipottie