I have always wondered if there is a cave complex under the hotel that connects to where the hot springs are located.
Could be that is where we get the water, and why it's so good for the skin.
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Some volcano water or something, rich in minerals.
It could be that there are some crystals down there, and that adds some fairy magic to the water. XD
In our world, there are several types of hot spring water, some are rich in sulfur, silica, calcium, sodium, or bicarbonate, each one claiming to have some different medicinal benefits.
So having something unique to this world, would make a lot of sense. It could be something where there are just rumors of water like this, that has some rejuvenating effects, that stem from those crystals (or others like them) being heated in the water deep underground, and infusing itself into the water and granting it mild properties. Maybe we could learn this from one of the fairies soaking in it, and being able to turn invisible while they are wet.