Answer
Montevideo lies on the shore of the Rio de la Plata estuary, rather than on the open ocean. The river carries large quantities of suspended particles—silt, clay, and sand. Wind and waves, along with shallow water near the shore, stir up sediment, giving the water a brownish or murky appearance.
Color alone neither proves pollution nor means it is safe to swim. After heavy rain, microbial blooms, or a health warning, water quality may deteriorate for other reasons. You cannot assess a beach by sight alone.
Before swimming, check the lifeguard flags and Intendencia de Montevideo reports on water conditions. If there is a warning, unusual foam, an unusual smell, or an accumulation of algae, choose another beach and do not allow children or animals to enter the water.