What will you do if the ship is delayed an hour returning to port? If you book a flight less than 5 hours after the scheduled port time for the ship, you're taking a big chance in missing your flight. If you buy insurance to cover missed flights, they will deny your claim unless you follow the cruiseline guidelines for departure times. Those guidelines are typically between a 4 and 5 hour window of time after the ship is scheduled to begin disembarkation.
Sitting in an airport with your luggage and trying to arrange another flight after missing your flight is both costly and frustrating. Not the way to end a vacation. Just my opinion (based on experience in the industry), book a later flight and don't take the chance. Also, it's not worth it just for the worry factor alone.