Layering Ocean Floor below water and beach above. How do you exclude region from affects of MaterialOnly brush?

Hello,

I am having trouble layering an Ocean Floor terrain (created using MaterialOnly custom brush) below water while maintaining the beach above. I would like to maintain the beach part of the Ocean Body

Any MaterialOnly affect I apply to the level seems to affect the beach regardless of layering.

This may be a bug or maybe I can exclude a region from affects of MaterialOnly brush?