Um, NO.
Cancun south to Tulum has better beaches and more beautiful water than you will ever find anywhere in Florida.
Have you been to the Caribbean coast of Mexico?
the water in the keys is very comparable to the water in cancun. snorkeling is better in the keys than cancun (and the isla mujeres) though. never been down to the tulum area for the water/beach though other than to xel-ha, but in general, while the water is nice in cancun, and the beaches there ARE nice, the beaches there are on the lower end of nice beaches in the carribean places i've been to. but like i said, they are still very nice. just not nice like aruba or turks and caicos.