How do I make this terrain tool ignore water?

So basically, I have a procedurally generated map made out of smooth terrain, and I'm making a terrain gun that can dig terrain, but I don't want it to be able to dig water.

In the built-in Roblox terrain tools when you subtract terrain this happens

But when you turn ignore water on it will erase all terrain besides water. ( I would show a gif but that feature is currently broken. )

Pretty much all I'm asking is if I have terrain in a 5x5x5 stud cube, I want all terrain in that cube to be erased except water.