Dehydration and procurement of water are key problems for vertebrates that have secondarily invaded marine environments. Sea snakes and other marine reptiles are thought to remain in water balance without consuming freshwater, owing to the ability of extrarenal salt glands to excrete excess salts obtained either from prey or from drinking seawater directly. Contrary to this long-standing dogma, we report that three species of sea snake actually dehydrate in marine environments. We investigated dehydration and drinking behaviors in three species of amphibious sea kraits (Laticauda spp.) representing a range of habits from semiterrestrial to very highly marine. Snakes that we dehydrated either in air or in seawater refused to drink seawater but drank freshwater or very dilute brackish water (10%-30% seawater) to remain in water balance. We further show that Laticauda spp. can dehydrate severely in the wild and are far more abundant at sites where there are sources of freshwater. A more global examination of all sea snakes demonstrates that species richness correlates positively with mean annual precipitation within the Indo-West Pacific tropical region. The dependence of Laticauda spp. on freshwater might explain the characteristically patchy distributions of these reptiles and is relevant to understanding patterns of extinctions and possible future responses to changes in precipitation related to global warming. In particular, metapopulation dynamics of the Laticauda group of sea snakes are expected to change in relation to projected reductions of tropical dry-season precipitation.