Your portapotty, your problem
Some marinas have portapotty dump stations, some will allow you to dump it down one of their restroom toilets...but nobody's gonna do it for you. The person who owns it is responsible for dumping it and has to find a place to do it. Some portapotties can be permanently installed...they're equipped with fittings that can be plumbed for pumpout, but unless yours is one of the larger 5-6 gal models, it's not worth the effort even if it is. Whether you can even fit one of the larger pumpable models in your boat is also an issue. Do you have an enclosed head?