There's plenty of good information in the archives
-- have a rummage around there.
Also there is no one right answer, because the right boat bike depends on how you ride, and how much storage
space you have. The small wheel ones (16" like the Brompton) with few gears are easier to store but not as rideable for long distances or rougher roads.
Kenomac is a really serious cyclist and I'm still kind of surprised he would be satisfied with the Brompton, but there you go. I'm not as serious a rider as he is, but I do like to stretch my legs a bit from time to time, so I prefer a 20" bike with plenty of gears, and with a suspension. By the time you get to 20" and you have front and rear suspension, then a folding bike rides quite a lot like a normal road bike and knocking out 50 or even 100 miles at a time is not a big deal. The big difference to a normal road bike is only in fast downhill runs, where the smaller wheels give a lot less stability, so you don't feel comfortable at the speeds you might ride your normal road bike downhill. However, the suspension makes up for the loss of resilience from smaller wheels; actually and then some, which is a nice bonus.
So my choice is a Dahon Jetstream EX with three speed front derailleur and nine speed cassette hub gear, and fully articulating suspension. It is definitely bulkier and harder to store than the smaller Brompton, though, and you don't need a 20" bike to run around the harbor, pick up groceries, etc.
I think of my bike as my land tender
, and I find it really useful, especially in more remote
places. I also have a lot of fun roaming around new places on it. But the downside is that it is hard to store, even on a 54' boat (!), and hard to maintain against the relentless onslaught of rust on board.
would make it even far more useful -- I'm looking at one like Ken's. Even if Uber has helped provision-hauling immensely.