Most waterfowl will not survive with this type of injury. It sounds like a snapping turtle may have gotten the foot. If you intervene, the possibility for survival in a captive setting is rather dismal as most wildlife rehabilitation centers do not have the equipment or staff to care for a swan with an amputated limb. If a rehabilitator can be located and the swan's condition is getting more dire, (i.e., the bird cannot enter or exit the water to preen or dry), then you might think about capture. Even then, the center may opt to euthanize. If the bird can enter and exit the water, even with some discomfort, it is best to leave it and if nature takes its course, then at least it is not having to be lifted by people and continually handled. The Regal Swan