River Lilly ??
I went Canoeing about 15 years ago on the River Trent I collected floating bit of greenery as it looked like a water Lilly.
For the first 4 years it grew and died back without flowering and was a drab pale green or light brown.
I decided to give it one more year and planted it in a chimney pot with the bulk of the plant close to the surface. The photo is what it did next it - grew right out of the water and since then has spread each year with floating seed pods and has a white flower to it.
Never seen one like it for sale in a garden center ! Sadly George the Belgian Shepard died a couple of years ago just before his 16 th birthday.