I too hunt in Minnesota and believe it or not I've shot 2 scoters (sp) before in this state. I talked to a CO and he said that some flocks get screwed up at times and end up in states where they don't normally belong. Turns out that 5 other groups reported shooting them too over the next 2 weeks in the same area so they must have stuck around for awhile but this was at least 5 years ago.
My 1st impression was that the ducks you saw where widgeons but if you are sure that they weren't then they could have been scoters or else some type of hybrid.