Human kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking seawater, you have to urinate more water than you drank. Eventually, you die of dehydration even as you become thirstier.
So, why the seawater is salty and the river water is not?
Rain replenishes freshwater in rivers and streams, so they don't taste salty. However, the water in the ocean collects all of the salt and minerals from all of the rivers that flow into it.
Is all rain fresh water?
The source of almost all fresh water is precipitation from the atmosphere, in the form of mist, rain and snow. Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled.
Why is there so much salt in the ocean?
Salt in the ocean comes from rocks on land. The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water). Sodium and Chloride are 'salty.'