I agree with the others that seeing Tulum ruins and also having any significant beach/snorkel time on Cozumel is ambitious for a Cozumel port of call stop. How much time will you have? If you were going to try it, I would suggest that you go with a taxi or two from Playa (not bus or collectivo) to the Tulum ruins. I think you would have to figure at least two hours for round trip taxi rides and I would think at least 2 hours, given catching the ferry etc, for round trip ferry rides. Many people swear by the collectivo's or buses but I think that can vary.