Everyone that keeps saying for the boss on the 17th mission to "Just lure him off the boat", or "Use the rain combo to knock him into the water", needs to explain how they're doing it. Every time I used the rain combo, my ninja would just grab him and slam him into the deck, not into the water, even if I'm right on the edge of the boat. As for luring him off the boat, he won't leave it, no matter what I do, and always stays on the middle part of the boat. I can't even finish the mission because of it.
Getting there without being detected is fine with me, since I figured out how to do it, and I've killed all of the enemies that are important in the area (so no back-up will come to help him, and so I would get a Ninja 5 rank), but he either kicks my ass, or just doesn't follow me.