We have a pair of folding bikes that we took on the boat, depending on where we were cruising - sure makes a difference in getting around by land if the terrain is conducive for biking. On the Erie Canal, we strapped the bikes on the radar arch...
Last month, while in Arizona, we picked up a pair of fat tire folding e-bikes...
Even folded, these would probably be too big to carry on a C-Dory (there are other, smaller folding e-bikes as mentioned in the OP). The e-bikes are a great way to get around. With pedal assist, you can easily do 15 to 18 mph, with a range of 25 to 35 miles. If you desire, you can skip the pedaling and just twist the throttle, top speed on this model is around 20 mph. (No idea of the range with just throttle, but I would guess it at 15+ miles).
It takes 4 hours to bring the batteries up to a full charge. We researched different brands and chose this model due to the low step-over height and the ability to get off the pavement. They are not lightweight (compared to our other folders), about 56 pounds (aluminum alloy frame, but the battery is about 8 pounds, the electric hub motor is over 15 pounds).
If you choose to not use pedal assist, there is no drag from the motor. Also (I get asked frequently), there is no regenerative charging (which would increase drag).
These aren't going to replace our scoots, but are a great in-town mode of getting around, assuming the traffic isn't crazy.
Jim