I played swordfish in a ladder game with blackwater cutlass on board. The card I dredged was a spell. I attacked with the weapon instantly ( no other interaction between the play of the card and the attack ) and the card I drew was an other one ( fog sail freebooter ).
It sounds like you drew a card when you attacked with the swordfish, which doesn’t normally happen, so I’m guessing you had a pirate minion in play that draws a pirate whenever your hero attacks