>>12573638Now, believe it or not, there is a river in this picture. I'm sure the wildlife living in said river aren't doing so good.
This is why I think Poison should be SE against Water. Pokemon, in their world, are living creatures. And those of them who are Water types, tend to (surprise!) live in the water and need it to survive. Corphish's dex entry even says it can't live in polluted water.
Oil spills and other toxic chemicals kill tons of water-based creatures every year. This isn't about poisoning the water, which as you pointed out, will dilute the poison. It's about poisoning the Pokemon who need the clean water to survive.