My game is a combination of a few different nautical settings, primarily Freeport but including the Shackles, and it's taken on a sandbox element recently. The PCs have a Letter of Marque from the Captains' Council empowering them to hunt specifically pirate ships, but being proper Freeporters they don't want to damage their reputations, so they've decided that they're only going to hunt pirates who openly defy the pirates' code and are generally disliked by even the pirates of Scurvytown.
They've chosen to use their time in this sandbox (aside from their personal ambitions and goals to which their hard earned gold will be used for) to go on a pirate hunting spree and I don't want for it to be a series of repetitive ship encounters.
What I need are suggestions for how to make these ships and their crews varied and interesting; what kinds of deeds and characteristics set them apart from regular pirates? Becoming hated even by pirates is quite a feat so I imagine these must be the evilest of blackguards.
I've only managed to come up with one which I'll give as an example, a ship called the Nightchild that plies Darklight Lane only in the dead of night, descending upon it's prey with almost supernatural stealth and swiftness. This unholy vessel is crewed by enthralled vampire spawn and their vampire officers. The captain is a dread vampire and a terrifying swashbuckler.
Not all of the ships need to be something extraordinary like that but having a wide variety of interesting crews would help me greatly.
Thanks for your input!
(If it helps, the party is made up of level 10 gestalt characters.)