I knid of disagree with mc's opinion of seawater here. Sure, crystalized salt is handy and drinking too much seawater is very bad. But if it's just replenishing your sodium levels if they are too low it works fine, and if you want to make it easier for yourself add the seawater to freshwater at a ratio of about 1:3. As for mc's second point about harmful micro-organisms, nothing prevents you from boiling it first before drinking.