The only cruise I have been on visited a different port every day for 7 days. I have always wondered what all the passengers do when they have a day "at sea". Where do 6000 people go? There are surely not enough lounge beds around the pool(s). Are there enough activities to occupy everyone? A friend told me that she spent the "at sea" days in the casino. (I can do that closer to home.) So what happens to everyone when they can't go ashore?
With one exception, the only cruises I have been on were jazz cruises. A promotion company buys out the whole ship, then hires musicians, then resells the rooms at a higher price, of course, because we need to pay for the music. The ships regular venues are all taken over with jazz; regular entertainment is suspended. If you are on the ship for the jazz, as I am, sea days are heavenly as you can go from venue to venue all day until into the wee hours.
I found the one nonjazz cruise I went on to be a bit boring on sea days.