I'm so glad someone posted this question. For years I have been hemming and hawing about doing an AI land vacation, but have always booked a cruise because I thought I would be board since I can only take so much sun / drinks.
I like the fact that on a cruise, I can hang out on my balcony in my pj's after dinner and watch the sea go by. Also, as the other poster noted, when traveling in the fall (cruise is cheaper then so that's when I go), the ship can move to avoid storms.
If I ever do try an AI, it would be 3-4 days only.