Myelin damage occurs in MS too. In fact there is a new theory circulating in the MS community that antioxidants may prevent that. Pycnogenols and grapeseed extract are good for this.
Some patients with RSD also use grapeseed extract, and I often do myself. This can really help prevent damage and encourage healing.

Any inflammatory process can damage myelin. However, there are systems to fix that. When damage exceeds repair, then there are problems. Statins for example... inhibit repair. This has been shown recently in studies.

As far as the water content goes, all our tissues have a substantial water content. It is one function of our skin to keep water inside us. Therefore poor fatty acid consumption...these are called ESSENTIAL for a reason.... leads to dry skin and water loss. Essential fats also help repair the myelin and cell membranes, and receptor sites. It is critical to consume Omega-3's (including flax) and Omega-6's (sunflower oil, soybean oil etc) in a ratio of 1:5 maximum in order for the protective and repair functions in our bodies will work properly.
We cannot LIVE without essential fatty acids, just like we cannot live without Vit C. Both of these nutrients have to be consumed with food. We do not make them in our own bodies.

edit to add:
There are also different forms of dehydration. Simple water replacement is the most common.

But people can become depleted of sodium (heat/sweating, some drugs), or potassium (diarrhea, some drugs, or poor diet), or both.
Low calcium is less common, because our bodies will borrow calcium from bone in a pinch. So rehydration may require use of electrolytes or not depending on the situation.