some catch on late to the whole nature documentary thing

I see this and I think: this photographer did something with his life.

Shark Spotted

Cape Cod, where you pay $20 to park at the beach, gets a shark!

The dorsal fin of the shark, sticking some 2 1/2 feet out of the water, was spotted 75 yards offshore at South Beach in Edgartown. Authorities received reports of other sightings along State Beach, on the island's northeast and the site of the opening scene of "Jaws." [link]

So here's my question, how do you really definitively identify a great white fin? Things I learned while trying to find the answer:

• Dolphin fins curved backward. Porpoises are stubby. [link]
• An example: The two white dots correspond to dorsal and tail fins.
  
• Along with teeth, man-eating, and a Jaws association, "e-How.com" tells us that the fin is remarkably large. This tells me nothing.
• And finally, the most awesomeist page on the internet: How to identify New England Sharks. The guy has a fun game here where people send in pictures and say "What shark is this, Tom?" and he figures it out. For instance, he identified this one as a Sandbar Shark:
  

Take home: I'm scared and fascinated by sharks. Happy Summer, folks.