The Florida Keys National Marine Sanctuary protects 2,900 square nautical miles of coral reef ecosystems, seagrass meadows, and mangrove habitats stretching from Biscayne Bay to the Dry Tortugas. Established in 1990 under the Marine Protection, Research, and Sanctuaries Act, the sanctuary encompasses the only living coral barrier reef in North America. Atlantic bottlenose dolphins inhabit these waters year-round, their pods navigating the Gulf Stream's warm currents that sustain over 6,000 marine species. The sanctuary's shallow banks and patch reefs formed over millennia as limestone structures colonized by elkhorn and staghorn corals, brain corals, and sea fans. Visibility often exceeds 60 feet, particularly during morning hours when tidal exchange brings clearer oceanic water across the reef crest. Key West snorkel and dolphin tour operators depart from Historic Seaport marinas, following regulated routes that minimize impact on sensitive habitats. Catamaran vessels equipped with swim platforms provide stable access to snorkel sites at Sand Key, Rock Key, or Eastern Dry Rocks, where angelfish, parrotfish, and Nassau grouper move through coral formations at depths between eight and 25 feet. Dolphin encounters occur in open water beyond the reef line, where captains track pod movements without pursuing or encircling animals. Federal guidelines prohibit feeding or touching marine mammals; observation occurs at distances that allow natural behavior patterns to continue undisturbed. The Eco-Discovery Center at 35 Quay Road serves as the sanctuary's educational hub, offering free exhibits on coral spawning cycles, sea turtle migration, and the historical shipwrecks that dot the reef system. Interactive displays explain how the Gulf Stream's northward flow carries warm Caribbean water at speeds approaching four knots, creating nutrient-rich upwellings that sustain the sanctuary's biological productivity. Morning departure windows between nine and eleven deliver optimal conditions for underwater visibility and calm surface water, before afternoon thermal heating generates wind chop. Sunset itineraries offer different rewards: horizontal light that illuminates the water column and increased dolphin activity as pods feed during the golden hour transition.