I just got a call from Gail M at 12:45 p.m. on Monday.  She and Barry C.
found 8 female BLACK SCOTERS and 2 SURF SCOTERS on the Potomac River
opposite Daingerfield Island.  Daingerfield isn't actually an island
(though it may have been in the past), but is the area just south of
National Airport and 4-mile Run and includes the Washington Marina. 
It's accessible from the George Washington Memorial Parkway.  You can
see the mouth of 4-mile Run from the marina, as well as a stretch of the
Potomac (all DC waters).  It sounds like the birds were about halfway
across the Potomac, viewable from the marina.

Good birding,
Paul Pisano
Arlington, VA