On a boat that small, stay with a self contained system
Replacing it with a marine toilet will require installing a new thru-hull for the flush water supply...at least one vented loop...the toilet will use at least 1/2 gallon/flush--compared to the cupful or 2 your portapotty uses, so you'd need at least 10 gallon tank to hold as many flushes...the tank and all the related plumbing will take up 1/3-1/2 your storage...and you'd be trading in a system that requires -0- maintenance for equipment and plumbing that require constant maintenance. If you're tired of carrying a portapotty tank off the boat to empty it, replace it with a model that's designed to be permanently installed and fitted for pumpout...and if you have enough height, one that holds 5-6 gallons--50-60 flushes...you'd need at least a 25 gallon tank to hold that many from a marine toilet. Thetford and SeaLand both have "pumpable" portapotties...Look for the "MSD" designation in the model name/number. Thetford even has an "electric" model that runs on AA batteries.