This chapter describes a model that can be used to predict hypothermia during cold water immersion. Drowning in cold water might precede the onset of hypothermia due to cold shock, injury or incapacitation. As pointed out in Chap. 129, there are three phases of increasing incapacitation leading to lethality – only the last involves hypothermia. Upon immersion in cold water, a poorly insulated individual will first experience debilitating cold shock that can lead to involuntary inspiration of water and subsequent drowning . This initial phase normally lasts a couple of minutes and subsides when the skin temperature plateaus just above water temperature. The individual that survives cold shock, or circumvents it because of adequate protection, is at risk of failure of limb motor function needed for swimming or treading water due to the cooling of the joints and musculature [2, 3]. The rate of limb cooling is governed by the temperature difference between the water and the skin, by any insulation between the two and by the cross-sectional area of the limb. Joint and musculature cooling increases with decreasing insulation, including the subcutaneous and fat layers under the skin, and with decreasing limb thickness. This cooling phase can last from 10 to 20 min for the average unprotected individual immersed in very cold water (0–10 °C). If death occurs during one of these two phases, the probable cause is drowning either through an involuntary excessive inspiration of water or due to a failure of the individual to initiate or maintain survival performance such as swimming, grasping onto floating debris or climbing into a life raft or vessel. The individual that survives these initial phases is then dependent on external devices such as personal flotation to maintain a head-out posture to avoid drowning. The risk to such individuals shifts to lethal hypothermia because of deep body cooling.