This actually is not wrong, it sounds like it's the Celsius equivalent of the 120 degree rule - if water temp + air temp is 120 degrees or less, you are at risk of hypothermia if you swim without a wetsuit. It's not so much a rule per se as a reminder that you can get hypothermic at *much* warmer temperatures than most people realize because water robs you of body heat so fast. So, say, if the water is 40 degrees and the air is 70 degrees, this adds up to 110 degrees and is unsafe even if the air feels warm enough to go swimming.
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This actually is not wrong, it sounds like it's the Celsius equivalent of the 120 degree rule - if water temp + air temp is 120 degrees or less, you are at risk of hypothermia if you swim without a wetsuit. It's not so much a rule per se as a reminder that you can get hypothermic at *much* warmer temperatures than most people realize because water robs you of body heat so fast. So, say, if the water is 40 degrees and the air is 70 degrees, this adds up to 110 degrees and is unsafe even if the air feels warm enough to go swimming.