Here is the view from where Dennis was sitting. The "toilet bowl" really does not show up well in this shot, even though it is near the center of the photo. There is a depression in the shelf just ahead (to your left ) of the white wave. The cave you see just to your left from the shelf curves around to form a tunnel which opens just above said depression. When waves hit the tunnel just right, water gushes around the curve and swirls in the depression, giving a "flushing toilet bowl" swirl effect.
This photo is marked on the "Map This" function, available from the button above the photo.