Cool, I was thinking of an erosion power for water as well.
I was thinking of a permanent with the name River, which has a (water cost) ability to destroy permanents.
Something like:
Cost: 6
absorb 3
Destroy 1 random enemy permanent per turn
OR
Cost: 6
absorb 2
ability: 2
destroy target enemy permanent
Hmm, too powerful. How about you exclude pillars?