Well, for one, Battle Axes were stupid heavy, and impractical when boarding ships. They're big, bulky, and good luck trying to take one below deck.
Now for defending ships, they'd be great, since you're already on the ship and not trying to move around a lot, they hit hard, and can even chop off grappling hooks or harpoon cables.
Though one can assume that pirates would mostly be on offense during their careers, so a lighter, more maneuverable sword would be the ideal weapon for raiding a target ship. You can still thrust and stab a dude in tight quarters (I.e. below deck, where the booty is), where a big, long, axe would be useless.
Though I don't doubt pirates didn't use improvised weapons, like hatchets or cleavers or whatever.
It wasn't bad, but it didn't live up to my expectations. It stands alone as an above averagemovie to watch but I expected something as good as the previous ones so maybe it's just me.
but you know, opinions and **** .