Yes, we all know you should drink water. It's good for your body and overall health. But, does drinking a lot of water really make a difference for our skin? Also, do thermal waters do anything other than feeling nice on the skin, or maybe a better question do they hydrate the skin from the outside the way water we drink is supposed to do on the inside?
Reason for me questioning this was pure curiosity. You always hear everyone saying how their biggest beauty secret is drinking water. If your skin looks grayish, drink water, if you didn't sleep well drink some more. Is that a myth or a fact? 
When you come to think of it, when you drink water it won't magically go straight to your skin. First it has to cross a long way to the kidneys and then it hydrates the cells. And drinking more water won't make your cells want to hydrate more. Moisture level of the cells is determined by genes, like everything else in our body. When it comes the scientific explanation it seems like drinking more water won't make your skin more hydrated. Still, when we drink more water we feel like our skin looks better, and we can actually see it does! 
This is probably because we feel better and when we feel good our whole body benefits from that. Also, if we are healthier our skin will look nicer. Maybe there is even a bit of placebo effect going on there. Bottom line is if it makes you feel good and stay healthy your skin will follow, it will also look and be healthy. 
And what about hydrating from the outside with thermal waters? I find that it does work. Although, you need to be careful. 
We all know that if we lick our chapped lips it will only make them more chapped. The reason is that the water that's evaporating also carries away surface moisture from our lips. Same thing happens with your skin when thermal water is evaporating. To avoid this, after you spray it pat it gently into the skin. Only this way your skin will have benefits from thermal water. 
At the end find what works the best for your skin and stick with that. If you feel like drinking more water is good for your complexion keep doing it. If you feel like using a thermal water is a waste of time don't do it. And of course don't forget to moisturize. Good moisturiser is after all a proven recipe for hydrated skin.