Once upon a time, "assassin" meant somebody who kills people for a living. Thanks to Ubisoft, it has come to enclose pretty much every vocation that involves the use of your arms and legs - small business owner, fencer, diplomat, thief, blacksmith, archaeologist, fashion model, big game hunter and, nowadays, sailor, with reality TV host and Olympic high jump medallist presumably on the menu for Assassin's Creed 5.
It's been a useful, beneficial process of expansion in many ways, accompanied by more intelligent, extended missions that draw on the full suite of quasi-Assassiny abilities, but one unfortunate side-effect is unwieldiness, inertia - a game that's got so much to offer it's obliged to spend hours tutoring rather than empowering you. In addition, Ubisoft's writers have become excessively fond of the grand, period-spanning narrative they've constructed, and dangerously unwilling to let players act out narratives of their own using that extravagant array of tools.
What does "pirate" mean? Well according to (Captain) Jack Sparrow it (more or less) means freedom. The ability to go anywhere and do anything you damn well please, come hell or high water (or both). A savage antipathy to any sort of restriction, however well-intended. The great hope with Assassin's Creed 4: Black Flag is that Ubisoft hasn't simply plucked freebootin' posterboy Edward Kenway from a historian's hat, but settled upon the so-called "Golden Age of Piracy" as a corrective to the landlubberly fussiness that's cramped the franchise's style in recent years.
The comparison outside the franchise is to Far Cry 3, a shooter built around similar open world tenets that's far more graceful and accommodating in its articulation of core conceits and tasks. Within the series, the natural reference point is probably Assassin's Creed: Brotherhood. The latter has been criticised for treading water with its story, but in some ways this was for the best - while Ezio muddled around in plot limbo, players were more at liberty to experiment with the abilities they'd picked up in Assassin's Creed 2.
Black Flag is more of a clean break from its predecessor, as any screenshot will tell you, but there are overtures towards Brotherhood's system-driven approach in the new ship battles, which in theory allow you not simply access to a sandbox, but the chance to structure the sandbox itself. That's thanks to boarding actions, which apparently do very impressive things with the second game's already scintillating water physics.
Once you've closed the distance to (and, in all likelihood, blown seven shades out of), your target you can order your crew to grapple it - whereupon they'll yank the two vessels together in real time. Factors like the violence of the weather and where you are relative to the other ship will affect the layout of the combat playpen thus created, and once you've let go of the wheel the options are myriad. You could swarm up the mast and rope-dart officers from the security of the other ship's rigging, or snipe using Kenway's blowpipe (for which there are several kinds of ammunition). Alternatively, you could lead your men in a glorious charge over the side, filling gaps in your swashbuckling cutlass combos with point-blank pistol shots.
The vulnerability of crewmates should impose an informal time limit on the mayhem - the longer players gad about, trolling the other side rather than getting the job done, the more empty bench spots you may have to fill later, assuming you've still got the manpower to make port. Prior to ship upgrades, at least, it may be wiser to scout out foes by spyglass rather than immediately challenging them, then whittling them down from afar with cannon fire before risking a boarding action. According to creative director Jean Guesdon, Kenway's floating home the Jackdaw will ultimately pack no less than five new naval weapons, and there's a new "trajectory based aiming system" which obliges captains to predict a target's movements before lighting the fuse.