Photo by me, Long Lake, WI
This plant likes still, shallow water. You can recognize it by the nearly round leaf. I say nearly because you can see a slit that goes almost to the center. It is native to the northeastern United States.
At www.nativetech.org I read that the roots of the white water lily were used by the Chippewa for a sore mouth. Additionally, "[t]he smoke from the rootstock is believed to repel crickets."