I personally think it would be better as a creature slot effect due to all the limitations you have to put on a creature version of Crater to make it reasonable. You'd have to make it immaterial (or burrowed) and you'd have to make it not attack and you have to give it 0 attack to begin with and you have to say that it cannot die for any reason (since Flooding and Crater would be Gravity, similar to how Ash is Fire).
It would be easier to make it a visual effect.