It's more of a question of preference. A little cooler than your body temperature (98.6° Fahrenheit) should not be too cold so that you don't get chilled. 70° to 78° Fahrenheit is the range where most folks feel comfortable swimming.
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What is the best water temperature for swimming in the ocean? There's no real optimal temperature, but many people feel “comfortable” swimming in water that is 70° to 78° Fahrenheit.
What's warm enough for you and what's warm enough for me may not be the same thing. But generally people will start going in for a swim when the water temperature warms up to around 70 degrees Fahrenheit. At 75 degrees, most people will venture in and at 80, anyone will swim comfortably.
The average temperature in Europe is 21.8°C. The highest current temperature on the continent according to our data, is in Koper (Slovenia) where water has 28.9 degrees celsius. On the other hand the lowest temperature is in Dalatangi (Iceland) where water has 12.1 degrees celsius.