Actual poison isn't the lifeblood of Poison-types that ain't like Grimer. Speakin' of, Grimer actually live in the water (ohai Celadon city), which shows the whole "diluted poison" logic really doesn't apply. Poison should be super effective against Water, with Water neutral to Poison IMO.
... Not that Game Freak will ever apply this change anywho. ._.