The need for fresh water may affect growth and survival of young waterfowl, including ducklings of sea ducks that are routinely raised in coastal marine habitats. We studied the influence of water salinity on growth and survival of Common Eider (Somateria mollissima) ducklings collected from nests on Southampton Island, Nunavut, Canada, in July 2001 and 2002. In 2001, 50 ducklings were partitioned into five groups (n = 10) and assigned a water salinity treatment for 6.5 days. Treatments included fresh water (<1 ppt NaCl), brackish water (11 ppt or 21 ppt), seawater (33 ppt), and a mixed regime (<1 ppt for 12 h, followed by 33 ppt for six days). In 2002, the experiment was replicated twice with different ducklings. Overall, growth rates were negatively correlated, and mortality rates positively correlated, with water salinity: 3%, 17%, and 60% of ducklings died in the <1-ppt, 11-ppt, and 21-ppt treatment groups, respectively; and 100% died in the 33-ppt and mixed-regime groups. The results confirm that fresh water is required for growth and survival of Common Eider ducklings immediately after hatch, which suggests that sources of fresh water in brood-rearing areas are an important requirement for successful reproduction.