Best Answer
As you know things shrink when cooled down. When you take an ice bath your hearth rate will initially increase due to the shock to your body but afterwords will go down. That is partially because blood vessels shrink causing the blood pressure to go up. Your hearth has to slow down to cancel that effect of increased blood pressure. The main reason is because tissues need less oxygen when cooled therefore less blood has to be delivered which in turn slows the heart rate.
That is why people who have drowned in extremely cold water have a good chance to survive even if rescued more than 30 min later