A friend of mine has 2 small ponds with koi and some battered up type of floating lily. Unfortunately I'm unsure of what kind. They aren't very hearty and the koi will eat the roots right up to the pad until they're all eventually dead.

Without any special sort of substrate what kind of lily could he use to shade part of the pond that would be hearty enough to handle the fish and do okay in a pond roughly 2' deep?

Thanks