Fitz, it sounds like you had some Ruddies swim by. Other than the thin neck part of your description they fit the bill - they tend to swim more than fly, they are reddish, hence the "Ruddy", and they are diving and popping up 20 yds from where they dove. They have invaded my honey hole and drive Jack crazy. He can't understand why we don't shoot em... They are a smaller bird than the broadbills but have a larger bill. Their wings are tiny, their feet are huge , their body is the size of a softball. It takes 50 feet to lift off when they finally take flight and when they land they just "plop" into the water, not gracefully at all. They are quite humourous to watch and drive retrievers wild swimming around your decoys. That's probably what you saw. Let me know if this sounds close.
“The perils of duck hunting are great- especially for the duck” Walter Cronkite