Water lilies create shade in the water, which is beneficial to both fish and the health of the pond or lake. Shade will offer protection to fish during the hottest parts of the day, as well as giving them a place to hide from predators.
Shaded water is also likely to have a reduced rate of algae growth. Water lilies help to keep the pond water aerated and clean, which will reduce the amount of time you have to spend on pond maintenance.