Concierge.com's insider take:
Celebrations worthy of a World Cup victory rocked the city when Cancún's beaches reopened in April 2006. Shortly after Hurricane Wilma struck, a Belgian company was hired to pump sand and re-create the coastline along the figure-seven-shaped Zona Hotelera. The results were amazing, with beaches far bigger than before. Unfortunately, phase two of the sand reclamation never happened, and the beaches are washing away. Sand is scarce in several areas, especially along the unprotected eastern coastline facing the open sea. Several excellent hotels like the Ritz-Carlton and Le Méridien have lost significant beach space and often have to rely on expanded decks and pool areas to keep guests happy. However, sand levels change with the seasons and the tides, so be sure to check on conditions before you book. The beaches are better on the north side of the zone facing Isla Mujeres and on the southeast coast near Punta Nizúc.
The sand that does exist isn't up to Cancún's previous standards; it's no longer talcum-powder white and cool on bare feet, but rather a pale tan with a few shells and some seaweed. Still, it leads down to what is some of the clearest, bluest water of any major resort area in the Caribbean. The beaches edging the southern leg of the Hotel Zone have the most wave action; Playa Delfines, for example, is a hot spot for local boogie boarders and body surfers. The beaches facing Isla Mujeres are bathtub-calm; of these, Playa Tortugas draws families with kids who zip around in WaveRunners or straddle banana boats (long yellow tubes pulled by a motorboat). Sea grass, which grows thick in summer, can create a somewhat slimy situation at northern beaches near the Embarcadero, such as Playa Langosta and Playa Linda.