score:6
While Europe swelters, members of the Austrian Ice Swimming Association spent three days in the Tyrol for training sessions in a kettle hole at the Nature ice palace inside the Hintertux glacier. Between 25 and 35 metres below the surface, the temperature of the glacier meltwater is -0.2C and the air temperature between 0C and 1C.
It really can't get any colder than that or it would be ice and not water.
Relevant links: https://www.tyrol.com/things-to-do/attractions/all-attractions/a-natural-ice-palace-hintertux-glacier
Upvote:6
Maybe this (in German) artical make give you some hint of where to look.
It is more scientific in nature than touristic.
Lunz am See plus a 3 hour hike in the mountains (1270 meters height) where areas can be found with tempature of 2 degrees, when in Vienna it is 30 at night.
No meantion of ponds or lakes directly, but possibly some exist.
Other, more touristic sites meantion the coldest lakes as between 18 and 20. KΓ€rnten with the most (1200) and Afritzer See, KΓ€rnten as a public see for swimming with 20 degrees.
https://www.urlaubsguru.at/reisemagazin/die-kaeltesten-badeseen-in-oesterreich/