Dehydrated skin can be treated really successfully. It is a good idea to look at all of the individual habits and life style to try and fix the problem.
I usually begin with what creams the individual is using. It is not enough just to correct this (if indeed it needs correction). If you are using the same cream for more than 2 weeks and your skin is still dry and dehydrated it is not working. Coming from a medical back ground, something either works or does not work. There is a lot of research to support that high % of vitamin C is really good for hydrating the skin. If very dehydrated use it more than once and day as hydration begins to come back into the skin you can decrease this to maybe once a day. Not one day is the same. Assess your skin each morning; some days you need more, some days you need less.
There has not been an individual yet that we have not managed to give them the tools to put hydration back into the skin. It may be a case where we have to add in hyaluronic serum or extra vit c mask. It is like having a tool kit for your skin. Depending on how the skin is each day you choose what is appropriate.
If an individual has a lot of dead cells sitting on the face (most of us do) a resurfacing peel would be required. Creams can not pass down underneath the skin if dead cells are blocking it. I am a firm believer in regular peels to keep the skin glowing and fresh. They have been around for centuries. They don't have to be invasive or require down time. Just enough to take off the dead cells. Clients who have experienced peels love the result and always come back for more.
To keep dead cells away in-between peels have glycolic/ retinol in your ingredients
Drink water - you can't beat hydration from the inside.
Dehydrated acne skin - hydrate with vit c but use a serum instead of heavy creams. Acne skin still needs nourishment but you don't want to block the pores to encourage more spots.
It's always good to look at what is going on in ones life if the skin becomes very dry. Different types of stress show in the skin; lack of sleep, alcohol, sickness, medications, major life events, pregnancy and so on can all have an effect on the skin.