They can be pretty.
I can't quite tell if the close-in of purple blooms in the photo is purple loostrife. The leaves don't look quite like purple loostrife -- and for your sake, I hope that is not what it is.
I got invaded by purple loostrife a number of years ago. It is awful. Every year I get as much out as possible by hand-pulling, using my backoe, and with aquaticic chemicals.
As for lilies, there are many that can be grown in sunken pots or mesh bags, and they will not spread and overtake a pond.
I am a big fan of water cleansing plants, especially in settlement ponds at the main water souces today ponds where it is practical. These settlement-capture ponds can become beauitful koi ponds with lots of cleansing plants.
I wish I had better answers,
Ken
