If planted directly into soil in a natural pond or in an aquatic basket in a mature lined pond with a good layer of sediment in the bottom your lily is unlikely to need feeding.  However, when repotting or in less mature ponds it is always a good idea to give them a feed.  Always use a good slow release fertiliser to ensure excess nutrients do not leach into the pond and cause/exacerbate problems with algae.  We use Osmocote Exact 5-6 tablets.