Swans might lose some of their foraging behavior because it expends energy which is used to keep them warm. Additionally, the food source during the winter months is different from the summer months (i.e. less of certain aquatic vegetation), and the different type of food source present may have more protein or filling properties which makes the swans feel fuller. This is especially true about cracked corn because corn acts as a filler while the pellets afford the swans with much needed mineral, vitamin and other essential micro-nutrients. It also could be that the time the swans eat is different than the time during the summer. If the birds stay on the water longer during the winter nights rather than during the summer nights, they may be foraging at night and not need to look for as much food during the daytime. Just a couple of thoughts.

Have you tried to use the kayack or boat solution for the swan at the sewer lagoon? The Regal Swan