There are tours from other sources if you are staying at the hotels in Playa, but I would think it would be easier to book through the cruise line. You can find info at Tourist Guide to Playa del Carme. It was an interesting place to visit.
I would suggest taking the ships excursion to Tulum. It is a long trip and you don't want to take a chance on missing the ship. If you are on the ships excursion and you are late getting back, the ship will wait for you.
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