I spend tons of time watching and photographing herons and egrets. One thing I have noticed is that when a very large fish is caught, or one that they have a tenuous grasp on, they will walk to land and swallow it there. That way if it slips out of their mouth it can't get away. In this case, the Great Blue Heron just barely had the fish by the tail, and walked over to the land, but for some reason turned around and went a little ways back into the water to swallow it. I wish I could ask the bird what the strategy was.