This sounds like a young juvenile who has been dispatched from its habitat by the parents due to the nesting season. If this is the case, it will eventually move on because if the bay is saltwater, it is not going to be able to drink and eat due to the salinity. Therefore, this situation should rectify itself soon as the young swan moves on.

Regardless whether it is a juvenile or adult swan, it may have also been dispatched from its habitat from hunting, predators or other pressure. In most cases, state and federal wildlife officials are trying to kill all Mute Swans in the U.S. so they can populate the Mute Swan's habitat with the larger Trumpeter Swans for Trophy Waterfowl hunting purposes. So, calling a state wildlife entity to find out if the swan is injured or ill, could result in the swan being intentionally killed by these same wildlife officials.

If you can find a state licensed wildlife rehabilitator that will go out and see if they might be able to capture and check on the swan, move it to a more suitable habitat, that would be the best way to approach this situation if the swan does not leave and the bay is saltwater in nature. Just make sure that whomever you call will not report the swan to state officials, turn it over to them or kill it. Otherwise, it would be better to let the situation play out itself. The Regal Swan