Okay, that’s odd. Both English and Scandinavian used to be closer on this, using the same original word “sea dog” originated in old Norse or so.

However both languages changed it to something else since that.

Both languages borrowed words from Dutch, but not the same Dutch word.

The English took “Schurk” for scoundrel and applied it to the fish, while Scandinavia took “Haai” describing the fin.

Curiously, Scandinavian also took Schurk and made it into “skurk”, but also uses “haj” as a scoundrel just the same as English in the word “loanshark” = “lånehaj” etc.

So the words have the same origin, but it was split in Dutch while being passed back and forth between languages.