Living in the sauna that is Arkansas, I don't usually think about dry air, but I think you are correct. If you've dehydrated yourself through breathing, plain water is fine for hydration.

^This person has witnessed Arrakis

If you can have food that is an electrolyte source with your water, that's fine.

If you consumed the food prior to the dehydrating events, it won't assist in restoring electrolytic balance.