Generally speaking, floating plants can be tremendously useful in a pond. They provide welcome shade for the fish, reduce the amount of light reaching the water and use up excess nutrients, thus reducing the likelihood of an algae bloom. Many also produce attractive flowers and leaf shapes.
Since many floating plants are not hardy, it is best to remove them from the surface of the pond once the temperature starts to fall. They can be over Wintered in a bucket in a frost-free greenhouse or conservatory.