Of course, there's no way of knowing, but I suspect a Pike. Not least because the slightly sad swan family lost a cygnet in mysterious circumstances from about the same point. Pike are ambush hunters; they dart and grab, but a grown duck would be big enough to break free.
Yesterday, the wind was blowing strongly on the river; rowers were catching the wind, being knocked sideways and blown downstream, making crazy times (in both directions). I made a joke about it with my boss, trying to be light-hearted (while we still can).