It turns out the ducks were diving down and prying masses of zebra mussels off the concrete wall below me, allowing for some excellent extended views. What is presumably the same bird has been seen here (Port Credit, Mississauga) very sporadically for a few weeks this December.
This Eider was not the first rare duck I saw that day. A male Harlequin Duck has been present all winter near Colonel Samuel Smith Park in Etobicoke, and shortly after sunrise I spotted him flying in with several Bufflehead. The ducks landed in a bay very close to shore, and I was able to get some very good views of this spectacular bird, although the early morning light didn't allow for great photos.
A first-winter male Harlequin Duck was present in exactly the same location this spring, and it seems likely that this is the same bird after having acquired his adult finery somewhere in Labrador or Northern Quebec.
This second-winter Kumlien's Gull was nearby. This is the best view I've had of this plumage, which is encountered less frequently than either adult or first-winter birds, but more commonly than third-winter birds.
One of my later stops that day was at a wastewater treatment plant in Oakville, which has been hosting several Yellow-rumped Warblers this winter. I noticed this Belted Kingfisher trying to hunt in the treatment cells - certainly not a good place to find fish!
Luckily it flew off back towards the lake as I walked around the area.