I know it may sound crazy. But it could be carp. I see them from time to time form a loose ball/school on top during the summer months. And if there is a free meal on top so much the better. When I was younger (and had extra time) we would actually fly cast to them from a boat & into the school trying to catch them. We caught a few. Real fun on a fly rod. We also would take a bowfishing rig & take them that way as well.

Not saying that's what you saw. But it could be 1 explanation.

Good luck & be safe!