The heron leaned over, touched the surface of the water with its bill and started opening and closing its bill rapidly. It did not stick its tongue in the water (tongue flicking) but kept this rapid opening and closing for about 30 seconds, then it went back to intent watching to see if this had attracted any fish. It only did this once while I was watching and it did not catch any fish this time. I researched later and learned this is called bill vibrating, a fish luring technique herons sometimes use.
Black-crowned Night Heron (Nycticorax nycticorax)