The entry, for those who are interested: http://pokemaniacal.tumblr.com/post/17760688728/stunfisk
Well, it’s true that most Water Pokémon do still take damage from Water attacks, but they do resist it. In much the same way, all Pokémon take at least some damage from Gust and Air Slash. I do think that Water attacks are, for the most part, using ordinary water (there are a few exceptions; clearly Bubblebeam is more than just bubbles, but trying to explain specific attacks is just asking for trouble) and that the damage is primarily in the force with which they fire it; for most Pokémon, it’s the impact of a Water Gun that’s doing the damage, not the water itself. For Pokémon that are weak to Water, obviously there’s no reason the water should be physically hitting them harder, so it seems likely that they are in some sense vulnerable to water as a substance. Fire Pokémon, obviously, do not take kindly to being dampened. As to the other two types, Ground and Rock… I vaguely recall reading a fan-fic once which suggested that Ground- and Rock-types possess a sort of exoskeleton made of a porous ceramic material, which can absorb a lot of damage (hence their traditionally high physical defence) but can become waterlogged, slowing them down and taxing their energy.
In short, I think that a Pokémon who is neutral to Water attacks simply takes damage from the crushing force of the water hitting it, while a Pokémon who is actually weak to Water attacks is somehow harmed by water itself, and has no business living an aquatic lifestyle.