Swim in Rīga
Rīga has many nice places to swim. Somewhere central and crowded? There's the beach at Balasta dambis 24, right by the Vanšu bridge. Then there's Lucavsala, across the Salu bridge past the Television Tower. Other places include Rumbula, lake Bābelīte, lake Ķīšezers, and so on. To get to the sea -- beaches are likely to be quite crowded, even though the sea is not yet very warm -- hop on a bus or train to go to nearby Vecāķi or Rīga's Bolderāja neighborhood.
Swim near Rīga
Ogre's Zilie Kalni park, some 30 kilometers from Rīga, is a nice place to swim. It also has a rather tall tower that offers a glimpse of Rīga's television towers and meanders of the River Daugava in the distance.
Swim across Latvia
For a weekend getaway, Latvia's eastern region Latgale - poetically referred to as "the land of blue lakes" - is an obvious candidate. Some well-known lakes in the area include Lake Rāzna - the country's largest - and Lake Lubāns, as well as Lake Drīdzis, the country's deepest, and Ežezers, which has the most islands.
And then there's River Daugava dividing the Latgale and Zemgale cultural regions--taking a dip is highly recommended.