Hawksbill turtles are considered Critically Endangered. An understanding of how nest microenvironment (moisture and temperature) and maternal characteristics can influence embryonic development, hatching success, phenotype, and hatchling performance is needed to ensure effective conservation management. We undertook controlled egg incubations at different temperatures and relative humidities. Additionally, we sampled temperatures at natural nests with data loggers, and relative humidity of the sand at nest depth, to determine the conditions that eggs experienced naturally. We varied relative sand humidity (RSH) percentages (30, 50, 75, and 100%) at a constant incubation temperature of 29.5°C. We also assessed constant temperature incubations at 25, 29.5, and 34°C with 75% RSH. Incubation at 29.5°C resulted in successful hatching (73.3%), whereas temperatures of 25 and 34°C prevented hatching. However, hatching in natural nests occurred even between 34 and 36°C, provided that eggs were exposed to these temperatures <20% of the entire period of incubation, and only towards the end of incubation. Controlled incubation at 30% RSH prevented hatching. RSH linearly affected hatching success, phenotype, and performance of hatchlings: eggs incubated at 100% RSH had the greatest hatching success and produced the heaviest, largest, and fastest hatchlings. RSH correlated positively with sand depth on the beach, so that ≥75% RSH is ensured at the average depth of natural nests (~39.2 cm), resulting in successful hatching (74.3 ± 34.7%). The controlled incubation showed that temperature and hydric conditions had the most significant impact on hatching success, hatchling phenotype (size and weight), and hatchling performance. Maternal characteristics, however, were slightly less important. These results suggest that the impact of moisture should be taken into consideration in hawksbill turtle conservation projects. The thermal and hydric environments experienced by developing embryos should also be considered when evaluating how climate change affects marine turtles.