A pretty tough one.
Though I agree that Kisame has nothing to actually kill Orochimaru, Samehada could become really dangerous. Not even Orochimaru is immortal once he's out of Chakra. Yet hitting him with Samehada could turn out to be a problem, thanks to his Kusanagi. It can't just extend like 200 meters, but was also said to cut everything. Enma was said to be harder than diamond, yet every single hit from Kusanagi caused enormous pain to him.
But to avoid that problem Kisame can create his water-dome and fuse with Samehada, thereby absorbing Chakra per mere skin contact. Absorbed Chakra can also be used to regenerate himself, thus making it not easy for Orochimaru to injure Kisame. Even though he could hardly get some of Orochimaru's Chakra, it can be absorbed from Orochimaru's snake techniques, like Senei Jashu oder Jagei Jabaku, since those snakes are living beings with Chakra after all. Still I don't see that much of trouble for Orochimaru in the water dome. He can summon various huge snakes, not just Manda, who will be of great help under water. Kisame's sharks are most likely too small to handle them efficiently. Though in this case his Chakra absorbing Daikodan no Jutsu could be pretty dangerous. In worst case Orochimaru could resort to Yamata no Jutsu. Apart from that Kisame's water dome might be blocked with Sanju Rashomon; if Orochimaru doesn't get caught inside, the scenario will just be like when Kisame fought team Gai, thus pretty unproblematic. The poison of Orochimaru's white snake form would be useless underwater though, since it only shows effect if his blood gets in contact with air.
To put it in a nutshell, Orochimaru wins by a close call.