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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Yes you can swim for a short time in water temperatures from 10 to 15 degrees without a wetsuit but you will be cold and you will get colder when you get out. If you are not acclimatised to the cold water, you will probably need a wetsuit for temperatures below 15 degrees until your body gets used to the cold water.