Okay - I've never been able to swim front crawl - but somehow 10°C water seems to make it work - probably survival instinct! It's October the 9th and in just two weeks the water temperature has gone down from 17.5°C to 10°C and probably lower because it was colder further out and I couldn't measure it there. The skies have been clear at night and and at 1400m altitude here at the Bozel "beach" it's been close to zero at night - with no clouds to stop the heat radiation from the water. In another week it's going to be a real challenge to swim here - but by the end of October the lake will be emptied for the winter. The body adapts quickly to the cold water as long as you go in regularly.