Rock shouldn't be weak to Water. The "geological erosion" excuse for Grass makes sense, because they magically accelerate years' worth of plant growth all in one instant. All Water Types do is spray jets of water. In no way, shape or form would that erode rock, not for decades or centuries. It makes sense for Ground to erode, but there is absolutely no goddamn reason a FUCKING SOLID ROCK would give a shit if it happens to get a little wet. Even a rock floor beneath a huge waterfall takes ages to erode.

And fuck off with the sandblaster comparison, that is a disingenuous and extreme use of water not remotely similar to the actual conditions Water Types fight in.