Pure H2O is absolutely necessary for all living cells. Almost all water that you are likely to consume, even what we consider to be "fresh water," contains ions (salts); however, many of these ions are also crucial for life (K+, Na+, Cl-, Ca2++ and N3- are some examples) and are non-toxic in the levels present in tap water, streams, lakes, etc. At high concentrations, like the concentration of sodium, chlorides and others found at sea, many of these ions and their compounds are toxic and can result in illness and even death.
<span>In response to high serum (blood) levels of various ions your body will simply try to pee them out. Drinking large volumes of seawater results in dehydration because the body uses more H2O trying to flush the salt out of your blood stream than it is able to retain. Drinking a small amount of salt water most likely will not hurt you, especially if you are already hydrated.</span>
