I easily found the male Harlequin Duck today, still associating with a small flock of Mallards. Viewing was easy, either across from the feed mill or climbing up the bank along Pennsylvania Ave. See the map below. The flock was easily spooked and would fly upriver for short distances before slowly floating back down.
Harlequin Duck location. The P signifies an easy parking spot and the blue area is where the Harlequin Duck was seen around noon. (click for full Google Maps)