So, this happened to me:
I think that boats and sailing around is pretty cool, but being able to miss important things by that close of a margin is really frustrating, and something like a telescope or some other tool to look farther out to sea than the tiny radius around your boat would make it a lot less frustrating.
Or just a bottle map or something that shows where Crab King is, some kind of clue or hint. Maybe even just something floating in the water in a larger area around him. That would reduce the frustration a lot, since I might have seen it when I sailed so close to him without knowing it.