My wife and I also fit your description of "reasonably fit early 60s" and we carry folding bikes when we vacation cruise. Our 20-something kids use long boards and can really cover some ground, but they grew up with skateboards and that's out of my comfort zone. I don't want to risk an accident, as we're not as resilient as we used to be. That would also be my concern with Razor type scooters, but they make some with bigger wheels which look to me more suitable for adults and might be a bit more stable (maybe not enough).

I've also seen some electric scooters that look quite appealing, but the problem with those is amps are precious when we're out for weeks at a time.( We rarely if ever tie to a dock longer than it takes to fuel up and pump out, and our solar budget is a bit strained when we're using lights and electronics, and opening the fridge a lot.). They also weigh more than the folding bikes, due to batteries and motors.

We have a Dahon Piccolo and a Dahon Mariner. They're each in the 26 pound range, and easy to one-hand into or out of the dinghy. They ride surprisingly well. If we're in one place for more than a day I lock them up ashore overnight.