Drowning in salt water is different than drowning in fresh water. Fresh water seems to be the more deadly variety, with more people drowning in fresh water every year than in salt water. Also, the time it takes for a person to drown in saltwater is between 5 to 30 minutes, while with fresh water, it only takes between 5 to 20 minutes.
In cases of fresh water drowning, the water filling a person’s lungs is considered “hypotonic” to the blood. The fresh water seeps its way into one’s blood stream where it dilutes the plasma/electrolytes, causing the red blood cells to swell up and eventually burst.
Drowning in saltwater is different. When the lungs fill with saltwater, it gives the opposite effect and becomes “hypertonic” to the blood passing nearby. The plasma in the blood gets sucked into the lungs and fills it, preventing gas/air exchange. Without oxygen, the heart gives up and one goes unconscious, eventually dying.