Imax1 said..
I now know...
up down fin = good
side side fin = BAD !
Also the thickness, sometimes the dolphin or shark aren't swimming, but just basking there.
Sharks have skinny dorsals, dolphins are fatties.
Seals, don't discount them, they are not all friendly.
I was surfing one day, just sitting on the board waiting for waves when a seal swam up to me to have a look. He surfaced just off the nose of my board, so I just sat there and started to talk to him, and no, I don't remember what I said, but it must have been something bad. He just stayed right in front of me, watching and listening to me, then moved up to the nose of my board and bit the crap out of it, leaving big canine teeth holes in my board, and swam off. The bastard!!!!! Ruined my day, had to go right in with a wounded board.