The thing was probably 8-12 feet and was schooling up fish. It looked like shark when it got into some fish and thrashed around but I don't know. Just the dorsal stayed up for around 20 minutes but a shark seems unrealitic.
If that fin stayed up above the water and moved in an s pattern it's a shark, if it was moving up and down then probably Dolphin. I have been around both. I used to surf alot and I have had a school of Dolphins playing in the water with me, I have also had sharks around, I had one actually bumped into me while I was sitting on my board, almost wet my pants! didn't stay in the water much longer after that.
I'm not an expert but I did grow up around and spend almost every day at the beach as a kid. As Steve said the way it moves through the water is a real good way to tell BUT ! If you see how that fin curves back like that it looks more like a Dolphin. It's not as pronounced as alot of Dolphins but looks more like one than a Shark. I use to fish off the jetty at Dana Point as a kid and we would see large Sharks come in the feed off the charter boat cleanings.
Yeah I thought the fin looked more dolphin like than shark but it definately didn't move up and down. There was an s pattern to its movements. the fact that it didn't move up and down made me question and that it was schooling up fish by itself. Thanks for the info guys