Convert Grass to Dirt Underwater Upon Generation?

Hi all. 
I'm wanting to replicate how the game typically turns grass into dirt when underwater. It's oddly confusing to me, so where could I start? I have tried surface rule conditionals with a type of water and an offset of -1, as well as a depth multiplier of -1, none of which seemed to work.
Would it be better/easier to try using disks for this purpose?