You CAN drink sea water. Your body would use both the water and salt. But, you just can't drink very much, and not without drinking something else, preferably nu e fresh water, to compensate and help it process the salt.

Like with most things, the poison is in the dose. Even at ypur most dehydrated, salt water from the ocean is too salty, way too salty, and not only will it wreck your kidneys, but without the other electrolytes beside sodium muscles and nerves won't work.