Over 30 people have drowned in Poland this month, including six on Saturday alone, according to police. Last year, 483 people drowned across the country.
The Government Security Centre urged bathers to swim only in specially designated areas, and never after drinking alcohol.
Temperatures on Monday were around 30 degrees Celsius in many areas of the country.
On Sunday, the mercury hit 36.1 degrees in the western town of Słubice, the highest temperature for two years, public broadcaster Polish Radio reported.
The heatwave is expected to continue at least until Wednesday, according to forecasters.
The hot and sunny weather has seen Poles flocking to beaches on the Baltic coast, to the northern Mazury lake district, and to local rivers and lakes.