Where did you find info on the amount of time in port? I've been looking but all the "7-day Western" info is for the Magic and therefore does not have Costa Maya.
I was in Costa Maya this past Feb on a road trip so we had a car. This used to be one of the MOST remote places in all of Mexico and so it's no suprise there's not much there. It's best to think of this as a beach stop. There are all manner of water activities (parasailing, jet ski rental, snorkeling/diving) beach massages, beach 'club' chairs with eager waiters. They have built a very cute boardwalk area with shops and bars (where the seats at the bar are swings) but otherwise there is not much here.