Thanks everyone. Answers to your questions:
I wouldn't do a conversion myself because I need the bike soon and I haven't any spare time at the moment - however a ready-built conversion from a reputable supplier would be OK.
Our boat is almost always in saltwater.
There will be hills because, no surprise, estuaries and harbours are at sea level so I'll usually be climbing for the first part of any journey. I don't mind assisting and as I said I'm reasonably fit but I don't have much stamina these days. On the other hand most journeys will be short, for example from the landing stage at Malpas to the Tesco in Truro is a 5 mile round trip, Itchenor to the nearest supermarket is about a 7.5 mile round trip.
Size and weight when folded are important. There's a reasonable amount of space on the boat but our tender is only 3.1m (see photo). There will only be one bike, for when I'm on my own, my wife isn't having one, and I can lash it safely, but even so it does have to be compact when folded. I'd love a Brompton but I'm not sure our budget will run to it, we are about to do our annual lift-out and there's some big expenses this year. Corrosion resistance is hugely important, friends of ours have had Bromptons for years, first pedal-only, now e-bikes, and they've never had a problem.
Thanks for the suggested models, I'm going to look at all of them.