Ducks at Santee Lakes

The "lakes" here are a chain of man made ponds that are part of an innovative water treatment facility. There are lots of fish (and fishermen) and lots of birds. We've seen ducks here that we have never seen before:

Two male wood ducks.

There are many Wood Ducks, and not in the trees. Back east they are a rare sight, and are often roosting in trees. Here they are everywhere in the lakes.

A pair of American Widgeons

A Northern Shoveler

A pair of Ring-necked Ducks

There are also Ruddy Ducks and Buffleheads, but they keep out in the center of the lake so I can't get a good picture.

???

Here I'd like some help from our readers. This is a VERY small grey diving duck and keeps to itself most of the time.  Before you say a grebe, note the bill - it is not a grebe bill, it's a duck bill.  There seem to be males and females, one with this kind of round cheek patch and the other with a faint stripe on the cheek instead.  I think it might be an immature Bufflehead. What do you think?