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It's simple, guys. Normal water for the most part doesn't outright harm most fire types or certain other Pokemon unless their bodies have a certain quality to them.
A fire type that would be visibly made out of fire or the like would have more issues with water than a fire type that doesn't have visible fire, or doesn't have the threat of dying if its fire goes out (Charmander line). However, even then they can deal with water.
Houndour/Doom, Vulpix, Growlithe, and many of those other Pokemon could deal with normal water just fine, and could likely enjoy taking baths or whatever else.
It's also likely that water doesn't really harm them at all. It just makes it more difficult for them to use their abilities and/or dampens them.