IIRC the theory is grass provides a substrate for aquatic plants and invertebrates to live on.
We just planted down about 2' below the eventual waterline, for erosion control before the pond filled. I suppose that's deep enough in most ponds to cover the light-penetrating zone in case there's anything to the above theory.
"Live like you'll die tomorrow, but manage your grass like you'll live forever." -S. M. Stirling