Black Swans are herbivores. They eat aquatic & marshland plants & algae. These birds also eat vegetation along the shoreline.
In shallow water, the bird dips its head & neck underwater in search of food. In deeper water, it up-ends itself submerging up to half its body underwater.
IT is also a filter-feeder sometimes running its bill along the surface of the water to capture floating plants & small insects.
We get a lot of swans on our local beach, they eat the seagrass in the ocean.