I've been past many sea snakes and sharks, but never encountered a shark with a purple fin. Is there a bug in my game?
Solution 1:
Just keep swimming and the purple finned shark will appear eventually
If you do not mind cheating, then you can get the game as text (on the "save"-tab), look for the entry bool gridItemPossessedPurpleSharkFin=false and change it to bool gridItemPossessedPurpleSharkFin=true
More here