SWIMMING AND WATER TEMPERATURE INDEX. 1  Here’s our full index we use to offer a guide to water temperature and comfort level for swimming. This is a guide only and in no way should be used to dictate how long to spend in water. 2  Water Temperature of the Sea Below 0°C (32°F) 3  Water Temperature of the Sea Below 8°C (46°F) For swimming this is considered very cold and even the most hardy among us would be foolhardy to venture in for a swim and risk hypothermia if exposed for any more than a few minutes.