Surfing offers unique challenges to thermoregulation and hydration. The purpose of this study was to quantify fluid loss in recreational surfers, and to analyze the effects of water temperature, air temperature, exercise intensity, duration, and garment thickness on the total amount of fluid lost during a surf session. A total of 254 male and 52 female recreational surfers were recruited from San Diego, Costa Rica, and Australia to participate in the study. Participants’ hydration status was assessed by comparing nude body mass pre- and post-surf session. Heart rate (HR), used as an index of exercise intensity, was measured throughout the session. Environmental conditions and surf characteristics were recorded. The difference between average pre-mass (73.11 ± 11.88 kg) and average post-mass (72.51 ± 11.78) was statistically significant (0.60 ± 0.55, p < 0.001). Surfers experienced a 0.82 ± 0.73% reduction in body mass. In multivariable linear regression, session duration and body mass index (BMI) were significantly associated with fluid loss. For every 10-minute increase in session duration, there was a 0.06 kg (SE = 0.001; p < 0.001) increase in fluid loss, and for every two unit increase in BMI, fluid loss increased by 0.05 kg (SE = 0.03; p = 0.02). Results suggest that prolonged surfing at high environmental temperatures in participants with high BMI’s resulted in significant body water deficits. Since there is no opportunity to rehydrate during a surf session, surfers must properly pre-hydrate before surfing in order to avoid the detrimental effects of dehydration.