Surprisingly, you can die from breathing in freshwater even hours after you have avoided drowning in it. This is because freshwater is more "diluted" concerning to ions than the fluid inside your lung cells. Freshwater doesn't cross into your skin cells because keratin essentially waterproofs them, but water will rush into unprotected lung cells to try to equalize the concentration gradient across the cell membranes. This can cause massive tissue damage, so even if the water is removed from your lungs there is still a chance you might not recover.
Here's what happens: Freshwater is hypotonic compared to lung tissue. When water enters the cells, it causes them to swell. Some of the lung cells may burst. Because capillaries in your lungs are exposed to the freshwater, water enters the bloodstream, diluting your blood. This causes blood cells to burst (hemolysis). Elevated plasma K+ (potassium ions) and depressed Na+ (sodium ion) levels may disrupt the heart's electrical activity heart, causing ventricular fibrillation. Cardiac arrest from the ion imbalance may occur in as little as two to three minutes.
Even if you survive the first few minutes underwater, acute renal failure may occur from the burst blood cells in your kidneys. If you drown in cold freshwater, the temperature change as the water enters your bloodstream may even cool your heart enough to cause cardiac arrest from hypothermia. On the other hand, in saltwater, the cold water does not enter your bloodstream, so the effects of temperature are mainly limited to heat loss across your skin.