This bird was found by Jim and Allie Loehlin, fellow birders in Duxbury, who alerted me. They recall that an Avocet was present in the same area in 2004 and stayed for a couple of weeks.
These distant handheld photos show the bird both standing and briefly swimming.
American Avocet - Duxbury, MA - Sept. 2, 2012 [1 of 4]
American Avocet - Duxbury, MA - Sept. 2, 2012 [2 of 4]
American Avocet - Duxbury, MA - Sept. 2, 2012 [3 of 4]
American Avocet - Duxbury, MA - Sept. 2, 2012 [4 of 4]