Most sea temperatures are less than 20 degree Celsius and most people in water have to combat cold. When considering human response to cold water a starting point is to consider human response to the environment as determined by metabolic heat gain being balanced by heat exchange by conduction, convection, radiation and evaporation. Nishi and Gagge consider the heat balance equation and thermal comfort criteria for people in hyper-baric environments and calculate significant increases in convective heat loss due to breathing with pressure. Heat transfer is mainly by conduction and convection, and although principles of human heat balance remain the same, avenues and properties of heat transfer vary from those in air. Stroke volume increases due to venous return to the heart and cardiac output and heart rate are lower than they would otherwise be in warmer environments. Muscle temperatures fall (from initial warm-up values), promoting anaerobic metabolism.