Freshwater is much more similar to the composition of our blood than salt water. When inhaled into the lungs, it passes into the blood stream immediately and thins the blood, leading to organ failure. All this may take no more than 3 minutes to happen and get the victim to death. 

The ocean waters take more salt than our blood. When salt water is inhaled, the body regulates itself for not taking water into the lungs, making the blood thicker. The whole process takes around 10 minutes, allowing a much greater chance of rescue. 

# paola 
Source: Mega Curioso