Add noise outside of a landmass using a mask?

Hey! any advice how I can add some noise outside of a landmass/continent on the flat area?

I do have a mask for the landmass, so I’ve identified it… just not sure how to get the rest of the way. where there’s low low areas and water, maybe add this mask in, and get some rock noise or texturing to make it like, more realistic water instead of paper flat

Feel like masks are a tough area so im taking notes and practicing.