Dry scalp? Shampoos and conditioners don't help, even dry after salon visit? Possible cause: dehydration?

Doubt it! I'm assuming your dry scalp is manifesting as dandruff. This is usually not due to any dehydration but is due to excess oil (sebum) production. You will need to use special medicated shampoo to control and avoid saturated or trans fats which can worsen things. Some people use tea tree oil which can help. It doesn't hurt to stay hydrated but that alone is unlikely to fix everything.