Anytime I absolutely, positively, need to be somewhere by a specific time, I always plan to arrive the day before. I have never been on a cruise, but considering the downside of missing the cruise departure, I think this guideline also applies.

If the cruise departs from a place you've never been to (or would like to visit again), it makes sense to fly in even earlier, enjoy a few days in the departure city, and have absolutely no worries about missing your cruise. At least, that's what I would do!