How to programmatically overlay a shader on a terrain by mask (Unity)?

I can paint the terrain with textures.I would like to see water drawn with a shader, and not a stationary water texture on the terrain. Moreover, it was possible to assign a shader to the terrain, but the problem is that in this case the shader is superimposed on the entire terrain tile.

How to write a code that would apply a shader to a terrain using a mask?