Definitely a Scaup, as for whether its a Greater or Lesser is another question as they're very difficult to distinguish and both species come through Southern Ontario in the winter. Shoveler's bills are very very large and difficult to miss. The colouration on this duck is also wrong for Shoveler (too dark overall - including bill - to be a female shoveler and lacking the deep rusty sides, white breast and green head of a male).
Iain