Swimming after Heavy Rain
I swim in an open pool. When it rains, it rains into the pool.
I had this experience - my body felt heavy in the water - requiring more effort, feeling of difficulty in stroking, sinking feeling.
I conclude that the "quality" of the pool water may have been affected by the rain. Consider the dust, exhaust residue, and so many other pollutants that may have commingled in the pool water coming off the rain water.
This may have caused the "heavy water" in the pool.