They appear in the campaign if the player is playing as a Vampire Coast faction. The player will be presented with adilemma to choose one of four factions to go after for varying amounts of treasure.
Once you have selected a faction, they will spawn a single army which is hostile to you and begin to sail around the map. You will have a quest to kill them in order to get your reward. Note that another faction may kill them before you get the chance, causing you to fail your quest.
So... it would appear that some enterprising fellows from the New World 'ave decided to risk it all against the elements an' of course ourselves, who surely 'ave nothin' left to lose anyway. It's looking pretty likely that Sartosa is their destination. Though they're well protected, taking their cargo as our prize presents an opportunity we daren't pass up.