Your feet and hands are the most prone to hypothermia. But having said that, when the body is immersed in cold water, entire body will lose heat. There are no specific places where the body will start to lose weight. But if you are exposed to the cold then your feet will start turning blue and then green and finally black. This is the last stage of hypothermia and there is nothing you can do except amputation. Avoid exposing yourself in cold water for too long and this is especially important for children. One the core temperature drops it is very difficult to bring it back up.