There are two native varieties, Frogbit and Water Soldiers. Frogbit is a petite and delicate plant that flowers, they are perfect for container ponds and small wildlife ponds. Water Soldiers are larger and more suited to medium to large size ponds due to the reproduction rates. Ponds with good stocks of Water Soldiers tend to not suffer with algae too much.
Other non-native varieties, such as Water Lettuce and Floating Watermoss are useful for offering shade and resting places for Frogs and Toads but are not hardy, so will die off once the temperature drops below 5 degrees.
All of these plants are sold loose, just simply place them on the surface of the pond.
Koi Carp, other carp and ducks will eat these plants.
For further information on planting your aquatic plants click here