We determined characteristics of the sand, level and frequency of tidal inundation, and hatching success at loggerhead sea turtle (Caretta caretta) nest sites on 8 low-relief mangrove islands in the northern half of the Ten Thousand Islands in southwest Florida. The sand was generally composed of larger particles and tended to be wetter, more porous, and more saline than on other loggerhead nesting beaches. More than one-third (38.9%) of the nest sites experienced tidal inundation. The mean salinity of inundating water was 26.9‰ (± 9.3, range = 0–40). The water content and salinity of the sand water at nest sites increased with increasing frequency of inundations. The mean particle diameter and total porosity of the sand were negatively related to sand water salinity, perhaps indicating that in the largest-grained, most porous sands, salt was more effectively washed out by rainfall. Hatching success decreased as inundations, sand water content, and sand water salinity increased. However, at nest sites that did not experience inundation, the sand water content and sand water salinity were not related to hatching success. Loggerhead clutches can tolerate a wide range of incubation environments, including a certain amount of inundation. Clutches that are deposited in low-beach areas (close to the water and more prone to inundation) can produce hatchlings. Because the characteristics of loggerhead hatchlings are influenced by the incubation environment, conservation strategies that involve moving all clutches from low-beach areas to high-beach areas may reduce the variety of incubation environments, thus reducing hatchling variability and possibly preventing the expression of characteristics that promote hatchling survival or otherwise increase the reproductive success of females.