Post-swim update:I should have known that despite being so much closer to the equator than Ireland, Gran Canaria is also right on the Gulf Stream, therefore the water temperature was still below anything I expected.
I went for the swim early in the afternoon, hoping the water would be warm enough. However, after the first 500m, I really started to feel the cold but managed another 500 before I experienced real shortness of breathe.
As a result, I disappointedly never reached my target distance on the day, so made the decision to swim the full distance over 10 weeks in the new year. What better