Ducklings and goslings are never left alone until fully feathered but they often get separated from their mother and siblings on their way to water after hatching. As a rule they will not survive long alone.
DO NOT place the baby in water. For the first few days of life they are not waterproof and very quickly become hypothermic and drown. Place the baby in a small box with a clean T-shirt or towel and contact a rehabilitator immediately. Please DO NOT handle it other than to place it in a box, these babies very quickly die from being handled.