The 20" wheel folding bikes were too big for our storage - and very awkward to carry. Definitely test ride a bike with 14" or 16" diameter wheels. To me, they ride just like a 20" bike.
We got two 14" wheel Genesis Commuters (which unfortunately are no longer available) and like them a lot. They only weigh about 30lbs and fold up into a small package which is easy to lift onto the dock or into a dinghy.
I noticed that the Jupiter Discovery has 16" wheels. If I was buying today, I'd look at that.
I would not buy a bike without a pedal for anything but riding around the marina. On any real hill, the electric motor in these things need a little help from a pedal or you have to get off and walk it.