On Saturday, the AAR reached 20 degrees in Bern, something that had not happened in May since 1975. Lake Geneva was at 23.6 degrees near Geneva. That of Zurich, at 22.4. Lake Murten reached 24.6. In total, 26 of Switzerland's 32 largest lakes record unprecedented surface temperatures for this time of year. The water of Swiss rivers and lakes is, in a nutshell, to get into.
In Spain, at those temperatures, everyone would be in the water without a second thought. In Switzerland, bathing in lakes and rivers is equally common, but it works differently. There is a whole culture around the "Badi", which are public bathing areas next to rivers and lakes: many are paid, have defined schedules, differentiated areas according to activity, and there are rules of behavior that should be known before going with the towel under your arm thinking that it is the beach of Benidorm.
You should also know that bathing directly in the river outside the authorized areas is allowed in many cases, but not in all: in areas of protection of drinking water or natural reserves it may be prohibited, and the signage exists although sometimes you have to look for it. Each canton has its own particularities.
For newcomers who want to take advantage of the heat: the most practical thing is to look for the "Badi" areas recognized by the municipality. They are usually well equipped, many are free or very cheap, and are the best place to understand how the Swiss live their summer on the water, which is quite different from how we do it.