It's a simplification of a traditional math puzzle question, so it's likely that some of your interviewees have seen it before. If you actually want to ask that specific version of the problem, those that remember at least parts of the solution have quite an advantage, if not it "only" gives them a good starting point on what to ask next to get clear details on the scenario.

Or you don't actually want to extend the scenario and a math answer (because which pirate ship is going to have a crew that thinks perfectly logical?) and confuse this subset terribly ;) Which might be the point of the exercise, but I'm not a good way of going about it.