You can just say "gracias, but no" and keep walking through the airport exit and you'll be fine.
If you want to experience PDC and points further south you really should stay down there for a few days to get the full experience then stay in Cancun. It's too far a drive to enjoy as you would staying there. You will find excellent advice on the PDC forum for resort or boutique options as well as excursions in the PDC area. And driving isn't bad If you learn the signs but do get full insurance covage either through your cc or out of pocket (most travel insurance and cc rental insurance is not recognized in Mex). PDC is a busy area now, points south (Tulum) are still quieter but getting busier.

When staying in Cancun we took the snorkeling/Isla Mujeres combo tour with friends last year and the snorkeling off the North shore of Isla was good.
We also took the 2-person power boats out of the lagoon a few years ago, through the mangroves and out to the reef for great snorkeling.