While Assassin's Creed III teased it with only a cinematic, you will actually be boarding other ships in real time in Assassin's Creed IV. A basic example I was told is that players will see a ship, weaken it with an attack, then command the crew to throw hooks onto their target in order to drag the ships together. From there you can board it, continue your attack, and take what you want.
"ACIII started the naval combat experience," Ashraf told me. "They did something that was really credible, really wonderful. But for us, that was really just a foot in the water. It was a tease of a potentially bigger system. We're diving really deep on this [with ACIV]."
The team started work on ACIV in the Summer of 2011 and knew early in development that the naval elements of ACIII would do really well. Their prediction came true as Ashraf told me that it was one of the most well reviewed aspects of the game.
"We kind of knew ahead of time that this would be very successful. We played the really early versions of [the naval element] and we felt like okay there's something really solid here."
Check out my preview on Assassin's Creed IV: Black Flag for more on the new entry to the series.