Looks big for a dolphin... but the wave probably acts to enlarge the fish
substantially.

Seems to me that the vertical fluke indicates a shark.

It would, but it is hard to tell if it is vertical. You can also almost make
out the sloping forehead of the dolphin. It is not unheard of for sharks to be
in waves like that, however. A surfer buddy who I forwarded it to has seen
such.