Berlin's water gauges log every lake's temperature each day. Pick a lake or river you are allowed to swim in, see how its water cooled through a whole winter, then tap the chart to colour every gauge on the map for that day.
Daily mean water temperature from the Berlin Wasserportal gauges (Land Berlin), downloaded 29 Sep 2026. Map outlines: Geoportal Berlin. A gauge sits at one fixed point and depth, so the water at your entry spot can be a degree or two off, and the surface of a still lake is often colder on a frosty morning.