My advice would be to have electric bikes that fold or are compact enough to store inside the boat.
I had a Trek mountain bike with disc brakes that I would store up on the boat deck, with "bike pajama's" covering it(down to deck level). However, after a few years the disc calipers became rusty (along with some other parts...)and basically needed replacing. I suggest avoiding external storage of bikes on a boat unless it's just for one or two trips.
Before I got around to doing that my house was burgled and the Trek, plus a Cannondale road bike, was stolen. So I'm about to search for a suitable electric bike to store inside, and I'll start a seperate thread on that in a little while.