If you want to go swimming in Kassel, Germany, then the Auebad might be your best choice. It is a combination of an indoor family spa with water slides, a sauna area, a pool for sports and an outdoor pool. The Auebad is located next to the river Fulda and the Staatspark Karlsaue. Swimming has a long tradition there as already in 1923 the mayor of Kassel, Philipp Scheidemann (yes, the one that in 1918 announced the German Republic) created a place at this location where it was safely possible to swim in the river.
The new Auebad was opened in 2013 and is still today a very modern facility. What I dislike about it is the location: afar from residential zones, best reachable by car. If you want to get there by public transport you need to catch bus 16 (or bus 13 that converts into bus 16) which is possible at places like the Stern, Auestadion, Altmarkt or Bebelplatz. Alternatives are the Kurhessentherme (which is focussed only on relaxation), the AquaPark at Baunatal, the Hallenbad Süd at the city quarter Nordshausen and in summer different outdoor pools.