pot it up and re submerge it providing it appears to have a growing point. if it is the older piece of rhizome it may develop a growing point.so you could leave it in a bucket of water to see if it will shoot. it will be fine in my experience. Many of my water lilies have been propagated by accident :o)

Thanks for the good advice. My husband got into the pond yesterday and unfortunately stirred up the sediment from the bottom of the pond. We only got the one water lily repotted as we could no longer see the others at the bottom. One done two to go.