It could go either way but I would give the edge to Itachi, Great Water Shark Bullet isn't going to work on Amaterasu, it appears where the user looks so the only way for Kisame to survive is make a clone and let the clone attack him with Water Shark Bullet which would hurt Kisame, and if water is able to extinguish Amaterasu why didn't Bee/Karin just go underwater for a while, not saying it is not possible but in my opinion it is unlikely but then again Kisame could make Samehada absorb it and use that water dome jutsu of his which could make him win.