We have a very small and shallow pond which is pretty neglected. We put a small water lily in it a couple of years back and this year the whole surface is covered in lily leaves and it has at least three flowers. I seem to remember reading that you should only have one in a small pond. What harm can it do? We do have a couple of goldfish in there.
Not too sure about the harm it will do but you can just remove some of the leaves without doing any harm and then in spring divide up your lily and give some to any friends you have who want them.