Nitpick mode:
Which
fin are you referring to?
The Dopefish has a dorsal fin, a caudal fin, a pair of pectoral fins, and a pair of pelvic/ventral fins.
I'm not an expert, but as far as I can see these are fish-like, not marine mammalian.
Also these fish come from Gnosticus IV, so who knows how evolution worked out there.