We all know that if you are stranded on a lifeboat in the middle of the ocean, you are not supposed to drink the salt water. Why is this the case--shouldn't the kidney be smart enough to retain the water and excrete the salt?
It turns out that the high osmolarity of seawater (usually >1000mosm/L) either approaches or exceeds the concentrating capacity of the kidney--and therefore you can't expect to retain any free water. In addition, seawater contains high concentration of magnesium and sulfate-containing minerals which can result in an osmotic diarrhea when large quantities are ingested; this can exacerbate free water loss which may likely already be high due to high insensible losses from wind and sun.
Some animals have impressive adaptive mechanisms to maintain homeostasis of osmolarity in a high salt environment;for instance, the albatross' nasal gland excretes a highly-concentrated salt solution, and the shark rectal gland has the same ability as detailed in another post.