Rock shouldn't be weak to Water. It makes very very very non-existent sense. Like actually think about it for a sec. Grass sorta works with the 'geological weathering' excuse because they grow plants in an instant. They are speeding up years of plant growth that would slowly fracture stone all at once. But Water Type moves are just fast-moving water. Solid stone takes ABSURDLY LONG to wear down, even if a waterfall erodes it 24/7.

Why would solid fucking stone care if it got wet? It would not. It would also help Rock out a whole hell of a lot.