Answer: The answer is both conduction and convection. Convection moves warmer water away from you towards cooler water and conduction transfers heat from your body to the cooler water.
Explanation: Water is a much better conductor of heat than air. So when you are surrounded by water, energy can flow from your body (conduction of heat) much more quickly than it would if you were surrounded by air at the same temperature. In fact, even if the water is warmer than the air, you might feel cold when getting into it.
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