The water in Stockholm is clean and perfect for swimming during the long days of summer. The best beaches are all west of the city centre: on Långholmens there's Långholmens strandbad to the west of Västerbron bridge, and rocky Klippbadet to the east of the bridge; and across on Kungsholmen, Smedsuddsbadet has a large grassy area for sunbathing.
Alternatively, Södermalm is the place to go for swimming pools; there are three in fairly close proximity: Forsgrénskabadet in Medborgarplatsen (t 08 508 403 20; Medborgarplatsen T-bana); Erikdalsbadet, Hammarby Slussväg 20, (t 08 508 402 58; Skanstull T-bana), which has an open-air pool; and the wonderful little Liljeholmsbadet, Bergsundsgatan 2, (t 08 508 411 77; Hornstull T-bana), a pool in a boat-like pontoon contraption that floats in Lake Mälaren. The last of these is closed from mid-June to mid-August, and has nude swimming for women on Mondays, and men on Fridays; the water here is always 30°C. For unofficial nude bathing, head out to one of the islands in the archipelago and find your own private spot.