I think that Luffy's battle with Caribou (yeah, they are going to fight) is going to have a lot of hype but not a lot of action. First off, unless the ships get hull to hull with the bubbles touching, he can't get on board due to being a Devil Fruit user. Second, Luffy now has something that can help in this situation, haki. The fight will last long enough for Caribou's Devil Fruit to be named and have a brief description, but that's about it. I don't even really think they will fight on the open sea like that, they'll wait for Fishman Island. I had this one funny image of Caribou seeing Luffy from a distance and starting to run toward him, slaughtering everyone within his reach, running maniacally up to Luffy, stopping, and then saying casually, "can I join your crew?" He's psycho, remember. It will be interesting to say the least. That's just my guess for what will happen. Prove me wrong or tell me otherwise.