A floating pond plant such as water hyacinths shade the pond surface, and are very beneficial in sunny weather and have the benefit of keeping algae in check.
These plants are free floating, rising above the surface as much as a meter in height with floating leaves and long, spongy bulbous stalks.
A floating water plant will also add another element to the garden ponds environment in that they bloom with showy purple to pinkish flowers and can also be divided and moved to other areas of the pond. When not in bloom this plant can be mistaken for "frog's-bit".
The water hyacinth is one of the fastest growing plants known, reproducing by way of runners or "stolons" that form daughter plants. The plant can more than double its population in two weeks, with seeds that are viable up to thirty years.
These plants thrive on toxins that are produced by fish, but are so pernicious that unless you are prepared to control them, they will take over your pond and starve the life out of it.
It blocks water flow, sunlight, attracts mosquitoes, starves the pond of oxygen and can cause a host of other problems. Control can be maintained by not allowing a large amount of excessive nutrients to remain in the pond. The plant in general, is considered a pest in areas of the Southeast United States, especially in Florida.
With "maintenance control" that state has been able to get these plants to a low level and keep them that way using bio control insects, herbicides and machines.
Please be careful when using these plants because, "a little goes a long way" as with all other elements of balancing a garden pond. Yes they will keep the pond cooler in warm climates, but too much can rob the pond of its other necessary elements in your effort to balance and help the pond survive.