Wow! I was surprised to say the least! I was in South Carolina camping along the Intracoastal Waterway in winter. At sunrise I was following both this Great Blue Heron north of Charleston and a porpoise feeding along the Intracoastal Waterway north of Charleston. My camera was focused on the heron when I saw some water splashing with a very upset bird! After reviewing the photo I knew for sure why the bird was upset. Also sharing the waterway that morning were a couple of porpoises. What you are seeing is an attack or at least a harassment of a porpoise on the heron. I would assume the porpoise may consider the bird to be an interloper for food in the waterway. But I've read some reports that these cute, friendly animals sometimes aren't so cute and sometimes are menacing with dark personalities.