Cancun is the most popular beach resort in Mexico. The beach is nice in general. However, the massive construction spoiled former charm of this place. Now many people prefer the nearby town of Playa del Carmen, and this is really good idea. Shore and seabed: sand. 
The best time for a holiday on the beaches of Yucatan Peninsula is a dry season from November to April. However, the summer half of the year is also good enough, since there are no crowds of tourists, and the sea water is warmer.