That is probably one of the better/rarer artifacts I've seen posted on here. I'd love to see some close ups of the blade edge. It sounds odd to think of people spear fishing for swordfish, but it's been a common way to hunt them for thousands of years. Some species come up to shallow top water at night and aren't apparently afraid of boats.
I saw a picture of a complete one found in a dredge, it looked like it was machined it was so perfect. Can you imagine all the non-gold treasures out there in the deep waters where those guys hunted.