If you tour the Mayan ruins, pack a BOB, Camera, bottled water, food, T.P., bug spray, hand sanitizer/wipes, sun screen, sun hats and sun glasses are just a few suggestions.
I went on a western Caribe cruise with Carnival that had an excursion to Mayan ruins. The people who were on the excursion got off the ship at Playa Del Carmen IIRC and the ship continued on to Cozumel. The folks on the excursion complained of no food, extreme heat/sun/insects and outragiously priced bottled water. Topped off by a sweltering bus ride down to Cancun to meet the ship.
For water activities, buy yourself a few disposable water proof cameras before you go. They are expensive to rent or buy on the ship. Book your excursions with the travel agent, the good ones like Scuba and Stingray city fill up fast. You can go snorkeling on your own, The ship will have gear to rent, at very high prices.
Also, pack your toiletries and a change of clothes for your first day on the ship in your carry on. It will be a few hours before you get your luggage. And remember, room service is included and available 24 hrs a day.