I can't say too much about the final because my hints may be jumping ahead of how you're thinking about the problem. This page mostly covers clarifications.
The problem statement says: "Murray, your chief cartographer, estimates that just following the currents will take you past exactly one plunderable town a day, for a total of n days... None could actually withstand a full attack, but some might do enough damage that you’d probably have to sail past a few towns while repairing the ship. So you need to decide which towns to plunder." Meaning: You're a pirate on a pirate ship, so you can't go back to a previous city because of the currents--you don't want to go against the current. (Metaphor courtesy of Connor) Conclusion: Trust Murray's schedule.