Secondarily marine vertebrates are thought to live independently of fresh water. However, some species of sea snakes require sources of fresh water even though they possess extrarenal salt glands. Here we tested the hypothesis, and demonstrate, that the only pelagic sea snake, Pelamis platurus, dehydrates at sea and spends a significant part of its life in a dehydrated state corresponding to seasonal drought. Snakes that are captured following prolonged periods without rainfall have lower body water content, lower body condition, and increased tendencies to drink fresh water than do snakes captured following seasonal periods of high rainfall. These snakes do not drink seawater. Snakes at sea presumably rehydrate by drinking from a freshwater lens that forms on the ocean surface during large rainfall events. These new data for spontaneous and voluntary drinking of fresh water, as well as the status of hydration and body condition, indicate unequivocally that this exclusively marine vertebrate dehydrates at sea and potentially remains in negative water balance for 6-7 months at a time.