If you dump toxins in a river, the animals and plants in said river have a tendency to die.
An excellent argument for why Poison should be effective against plants (which it is) and animals. However, the water itself is still fine.
Poison is diluted by water, but poison also contaminates water. It works both ways hence the normal damage on both ends.
Apparently water == Pokemon now.
ALL Water-type mons are Water.
Your logic would only work if its something made of water. None of the Water mons are made of Water(well...except Tenta). They're just animals with an affinity for water.
Exactly they should be able to use their natural affinity for water to easily dilute the poison. I mean if you want to use the logic that if you poison water you kill the animals in there you get on a pretty slippery slope.
its tough to argue either way because Gamefreak isnt consistent.
Grass beats water because it.. drinks it i guess? But not really a threat to the animals. But electricity is super effective to animals, right? because water itself is a bad conductor but the salt from a squirtle or something is what really makes it effective.
so poison could go either way. based on the fact that poison sucks and water doesnt, C