Metabolic rates and rectal temperatures were continuously monitored for humans immersed in cold ocean water (4.6–18.2 °C) under simulated accident conditions. The subjects wore only light clothing and a kapok lifejacket while either holding-still or swimming. While holding-still, metabolic heat production (Hm, kcal∙min−1) was inversely related to water temperature (Tw, °C) according to the equation Hm = 4.19−0.11Tw. This thermogenic response pattern is shown to be similar to that for exposure to air of the same temperature when air velocity is just over 5 m.p.h. (2.24 m/s).The thermogenic response was one-third efficient in balancing the calculated heat loss in cold water, resulting in hypothermia at a rectal temperature cooling rate (C, °C∙min−1) dependent on water temperature (Tw, °C) according to the relation C = 0.0785−0.0034Tw.Although swimming increased heat production to 2.5 times that of holding-still at 10.5 °C water temperature, cooling rate was 35% greater while swimming.A prediction equation for survival time (ts, min) of persons accidentally immersed in cold water (Tw, °C) has the form ts = 15 + 7.2/(0.0785−0.0034 Tw), based on the findings of this study, and it is compared to pre-existing models.