We're talking about way up at the northern tip of the park near Bear Cut, the swift-moving channel that separates Key Biscayne from Virginia Key. This is the very essence of serene. Outside of adventurous nudists and the occasional school science class trolling for sea specimens, few people explore this last vestige of Miami's natural coast. Get there early on a bright morning and you'll think you've stepped into a travel brochure, shuffling through virgin sand and splashing in crystalline water. Looking to the east you'll see nothing but open sky and ocean. Turn your gaze northward and you can pity the poor souls trapped in traffic or attached to cell phones in the big city. But move quickly if you want to enjoy the place. Fashion photographers increasingly use the beach's vistas and lush backgrounds, and the county parks department recently completed a boardwalk that will deposit too many visitors.