We cannot open your video, but what you are describing sounds like bathing behavior. For observers seeing this behavior for the first time, it appears that the swan is flailing and in distress. The swan will go under water with its neck flopping water back up and onto his back and scrub his feathers with his beak. This is how the swan gets all of his feathers wet. Once he has had a good soak and rinse, he will return to the bank where he will begin to let the sun and air dry his feathers. He will shake to remove exceed water, use his beak to zip his feathers, similar to Velcro and then run his head over his rump. The rump is where the uropygial oil (preen) gland is
located. The oil from the gland keeps his feathers healthy and water repellent. Without this oil, the swan would sink and drown if his feathers were not water repellent. Some research suggests that the oil acts to keep harmful bacteria off the swan's feathers and body. The Regal Swan