Drowning refers to death by suffocation from immersion in liquid. From there, the technical aspects differentiate drowning into different classifications. There’s dry drowning, which occurs when your larynx snaps shut like a trap causing suffocation. Then there’s the wet kind of drowning, and it turns out that the cause of death is dependent on the type of liquid that the victim drowns in.
Fresh water is hypotonic compared to your blood, meaning it has less “stuff” in it like electrolytes, proteins, cells, etc. Because of this fresh water is immediately absorbed into the blood as it enters the lungs. Once in your blood stream it dilutes everything rapidly. This causes your red blood cells to burst, dumping all of their goodies into your blood stream. This dilution lowers the concentration of the electrolytes sodium and potassium causing your heart to freak out to the point it stops; the cause of death is ventricular fibrillation, not asphyxiation.
Salt water has the same tonicity as your blood, it’s not as quickly absorbed into your blood stream and it doesn’t cause your red blood cells to pop like balloons. The water mainly stays in your lungs, so the cause of death is asphyxiation by pulmonary edema (you can’t breathe because your lungs are full of fluid).
Source: Compound Fractur