The phrase "in the pond" would seem tyo be the problem. It will grow much better in moist ground at the edge of the pond. We are growing it, successfully, on the site of a filled in pond and it is not actually very moist at all.

I tend to agree Bulbaholic. We have one in our pond and i must admit it is quite often more out of water than in. It's about 10 years old now and just gets bigger and bigger. I have never re- potted it! It has a mass of flowers every year. The frogs love it!