There are many good sources on the web to search good questions like this. oceanservice.noaa.gov gives a good answer. It basically says that the salt content in sea water is too high for our kidneys to handle. Our kidneys filter excess nutrients, like salt, and urea from our blood, but they can only make so much urine at a time and only while the blood is hydrated. So basically, the human body cannot expel enough of the salt in sea water in time to make the blood healthy again because the salt content in urine can only be so much.
