21 degrees is warm

Well, if you have 21 degrees air temperature, it's ok if the water you'll swim in is warmer, but if the water is cooler then it won't feel very comfortable. If you mean 21 degrees water temperature, it's ok to swim if the temperature of the air is warm (otherwise you'll feel cold when you come out). A heated pool's temperature is around 30 degrees, so 21 is not the same, but it's perfectly ok once you go in the water.