The official swimming season in Latvia starts on the 15th of May and ends on the 15th of September. According to the information provided at the Riga City Council webpage, there are official swimming places located around Riga, where the quality of water is controlled by the Ministry of Health of the Republic of Latvia.
The map represents the official swimming places in Latvia and in Riga in particular, while via this link you can see the monthly reports on the quality of the water in these places. Information is available only in Latvian.