Maybe you do...and maybe you only THINK you do...
With a 26' boat, you're right on the borderline between whether a fancy portapotty or a head and holding tank is the best way to go. If the portapotty is one of the larger (5-6 gal) "pumpable" models, you may be in better shape than you would be with a "real" toilet and a holding tank...'cuz it uses so little water that holds as many flushes as "real" toilet puts into a 12-15 gal holding tank, plus it's all self-contained...whereas a holding tank is gonna take up quite a bit of otherwise useful storage space. Then, unless you're on a fairly clean inland lake, there's the matter of sea water intake odor...hoses that can permeate with odor and stink...and preventive maintenance on the toilet. Even if your current portapotty isn't one of the larger "pumpable" models, one will fit in the same footprint...Thetford even makes an "electric" model that runs on 6 D-cells!  And you can still install a y-valve and pump to dump a pumpable portapotty at sea, so if I were you I"d give it a bit more thought.However, if you're determined...a head and holding tank aren' t too difficult to install. Unless there's already one there, you'll need an intake thru-hull & seasock for the head flush water intake, a pumpout fitting on deck, and a vent thru-hull for the tank. Some hose, a y-valve so you can dump it, a pump, another thru-hull and seacock to dump it out of, a vented loop between the pump and that thru-hull...and, of course, the tank and the toilet. Total cost: $500-700. We can help you with the tank...as for the toilet, I'm partial the Raritan Cricket, but the manual Jabsco toilets are on sale right now for $99...for that price, I'd buy two and keep one as a replacement instead of rebuilding. Give this some more thought...and if you're still convinced you want to do it, I'll be glad to help you with the installation.