Stockholm city build on 14 islands and there are many great places to swim and relax on sand beaches under the sun. Some swimming beaches are so near in the city such as Smedsuddsbadet and Långholmsbadet.
Further you may have a wonderful city views from these sand swim points backed by natural beauties. These spots are very crowded during summer days and especially on weekends. You may explore further swimming spot in outskirt Stockholm.