One look at Cancún's aquamarine Caribbean waters and it makes perfect sense why planners back in the 1970s were so eager to develop the area as Mexico's next big resort destination. With about 19km of powdery white-sand beaches in the Zona Hotelera and a quieter 15km stretch of coast north of downtown, Cancún is a beach bum's haven. You'll find some of the most swimmable waters on the Zona Hotelera's north side, between Km 4 and Km 9, while north of Cancún Centro, Isla Blanca beckons with its long stretch of relatively undeveloped coastline.