I went around and around with this a few years ago. There are a couple of important reasons why everything from mopeds to bicycles didn't work for me. I did have a Diblassi folding motorcycle at one time. If you want to spend all your time in a marina, that would ease the issue of on and off the boat...to some degree, but not entirely.
I bought adult model Razor scooters.
After getting them, I ran across a couple of used Gopeds. I decided that I would give them a try as well.
For me, the Razor scooter just really fit the bill. One huge plus for me is that I never have to worry if they will stolen while I am inside a store. Always always folds up and goes inside with me.
I did sell off one of the motorized Gopeds. The buyer has reported back that he is getting all over Baja
and the Mexican mainland with it.
The jury is still out on what to do with the second Goped. In the meantime, there is room for it and the 2 Razor scooters aboard.
I vote against Mopeds.