Maybe it's a tub with no top or bottom and a net across the bottom, and you pull it through the water? If it didn't have a top or bottom I don't see how it would do anything. Maybe that's how it was done before cast nets became popular around here.
Well anyways nice detective work. I searched Google for tubbing, tubbing fishing, etc and found zilch.
If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away.