In my first aid kit I carry* sterile normal saline for wound and eye irrigation (most often used for the latter). Can you drink it? Would it help with hydration? It's nominally isotonic with blood serum, but I'm not sure what effect it would have if consumed.
What got me thinking was a slight planning error in obtaining water on a bike ride on Sunday. It was nothing serious but I was a little dehydrated with only a mouthful or two left in my bottle before I found anywhere to get more water up and I was starting to worry. I tend not to finish the last bit in my bottle unless I really have to, saving it in case of things like dust/insects caught in my throat.
* only 2×40ml vials in my light (cycling) kit but in some situations I might have more, so I'm thinking about concentration not quantity