I've seen lots of bikes on yachts and larger sailboats, as they tie up in a marina. Lots of retired or wealthy folks who have taken up the liveaboard boating lifestyle, travel from port to port and marina to marina with their bikes. Once docked, they hop on their bikes and go exploring to restaurants, etc.

So when I first saw the title of this thread, I thought it might be one of those liveaboard couples with their bikes travelling the world by boat.

The only thing that really comes to mind is, as much rust and corrosion that results from being down on the water in the salty air, plain steel bikes are soon toast. Titanium lasts the longest (forever, best as we can tell). Aluminum - depends on the alloy.

Bikes at sea certainly can make sense...