More than likely, the swans were flying through your area and we're trying to cool off. Once they rested and cooled, they went on their way. You are correct in that most swans need freshwater habitats. There is one species, the South American Black Necked Swan, which can survive in brackish water because they have a filtering gland that displaces salt. All other swans cannot survive for a long period in salt water or brackish water. The swans may have been chased from their area due to hunting or predator pressure and used your water for a rest/cooling respite. The Regal Swan