If you are stranded in the middle of the ocean for a few days, why would drinking salt water NOT be a very goo?

It's hypertonic, meaning that it has a lower water potential than that of your body. Water goes from areas of high water potential to areas of low water potential (down the concentration gradient). This is in order to achieve equilibrium, meaning basically that it tries to get the same water/solute ratio all over. Water from your body and cells will diffuse into the salt water in order to achieve equilibrium, thus dehydrating your body and brain.