Can someone help me identify this water plant?

We found a plant last year while mushroom hunting on some of my family's land, on the top of a hill, in the middle of nowhere, in the kind of place that rarely sees people. I don't know if Native Americans planted it there way back when or if it just came up there. My great grandparents may have planted it for all I know. It was growing in a boggy oasis-like area in about 6 inches of water. It was blooming then. I don't remember what time of the year it was. Anyway, the plant looks like a hosta mixed with a peace lily. If I remember right the bloom was fairly large and snow white. We were in the same area again the other day and the plants had survived the winter (Kentucky, zone 6) and had spread, so I dug some up and brought them home with me. I intend to put them in the shallow end of my pond. However, I was wandering what they were and if they were harmful to my koi. The water they were growing in didn't have any fish. Are they a common plant that anyone with any sense should have recognized or are they something “new”?

See picture at

http://i299.photobucket.com/albums/mm320/snailbisc...

Thanks!!!