The U.S. Geological Survey (USGS) Coral Reef Ecosystems Studies (CREST) project (https://coastal.er.usgs.gov/crest/) provides science that helps resource managers tasked with the stewardship of coral reef resources. Coral reef organisms are very sensitive to high and low water-temperature extremes. It is critical to precisely know water temperatures experienced by corals and associated plants and animals that live in the dynamic nearshore environment to document thresholds in temperature tolerance. This dataset provides underwater temperature data recorded every fifteen minutes from 2009 to 2020 at six off-shore coral reefs in the Florida Keys, USA. From northeast to southwest, these sites are Fowey Rocks (Biscayne National Park), Molasses Reef (Florida Keys National Marine Sanctuary, FKNMS, site terminated in 2013), Crocker Reef (FKNMS, site added in 2013), Sombrero Reef (FKNMS), Pulaski Shoal Light (Dry Tortugas National Park), and Pulaski Shoal West (Dry Tortugas National Park, site added in 2016). A portion of the dataset included here was interpreted in conjunction with coral and algal calcification rates in Kuffner and others (2013).