Does "hydrate" add some nuance here that "fill" doesn't? If so, I'm missing it.
Also it just sounds cooler.
Also, as I mentioned in another comment, this phrase doesn't work for the metaphor. Dehydrating and rehydrating a lake isn't a good idea; it'd kill the ecosystem, and it'd be tremendously expensive. I'd rather stick to dehydrating and rehydrating something else, like fruit. Here, this looks similar to the fruit soup my in-laws make: https://www.cheaprecipeblog.com/2015/11/norwegian-sweet-soup...
in my experience it's not just the untransformed ones that are like dead fish in a hole.
I also smirked at the metaphor. The AWS marketing department must be high fiveing right now