i actually think this is kind of an interesting question. i want to say that it's possible for fire pokemon to go into water, and i'd argue that that, at least with the more animal-esque ones, they can probably swim competently, but it'd just be really uncomfortable.
as far water pokemon dehydrating, they probably can? as a wild guess, anyway. they probably find higher temperatures without some form of hydration unhealthy for them so they would need to hydrate themselves as well, even if they have water stored in their bodies.
interesting things to think about. @
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