I saw a great sea behaving slightly oddly yesterday at Pegwell Bay, Kent. It was splashing around, throwing itself up to the water's surface, showing its hind flippers, rolling around and creating lots of splash. It would then dive down, swim slightly up the river, then do the same a dozen or so metres upstream. I don't think this was a warning/alarm signal to me as I don't think it would've seen me, and was doing it before I got there.
Any ideas as to what this behaviour might be?