In Bremen, there are several nice opportunities to go swimming without paying an entrance fee. In Bremen there are 10 swimming lakes and a little beach at the Weser.
There are rules for bathing you have to follow. (Available in many languages)
Once in a month there are checked for the quality of the water by the local health authority. The quality of the water, temperatures and more current information can be looked up under the following link: http://www.bauumwelt.bremen.de/umwelt/wasser/baden_in_bremen/aktuelle_temperaturen_der_badegewaesser-48224 (German Language)
Radio Bremen offers a map of all swimming lakes in Bremen and the surrounding area.