Alright I'm seeing a lot of different opinions so I'll state my reasons why I think the Suiton users win this. Anyone feel free to counter this so we can get a discussion going
-Tobirama can mark Kisame so the two can teleport between marked locations
-Kisame can spit out his water dome without endangering his teammate since Tobirama can teleport outside the dome
-Tobirama and Kisame both know not to look into Itachi's eyes so they can avoid eye contact - both Tobirama and Samehada (Kisame's sword) are sensors so avoiding eye contact is something that's actually possible for them
-Kisame can absorb Amaterasu when he's merged with Samehada; Tobirama can sense it coming and teleport to dodge it
-Orochimaru's snake summons can be merked by Kisame's 1000 sharks, or he can fire off a GSB at the summon and drain it of chakra; when their chakra hits zero, they die. Same with 8 Branches
-Kisame can make water clones and have the clones merge with their duplicates of Samehada to f**k shit up in close combat. Itachi can't waste his chakra on clones so he won't use Amaterasu on them; Orochimaru will have difficulty taking down all the clones, especially if Tobirama marks them (Tobirama can also mark Kisame's sharks)
-Itachi's Susano'o is taken out with Great Shark Bullet. Orochimaru and him end up dead through drowning, chakra depletion, or shark feeding frenzy
Dome wouldn't be helpful. For one, it'll make Tobimara rather useless at that point with his suition jutsu. Also, having ore than one opponent, Kisame would have to choice one to follow as the dome follows Kisame's movements. Orochiamru and Itachi would only have to stratgeize which would be a decoy for the other to escape. In this scenario, Orochimaru has the jutsu to supply distractions, such a 10,000 snakes and/or large snake summonings. Orochimaru can easily escape this technique w/ attack prevention technique.
Avoiding eye contact leaves either vulnerable to Amaterasu. While the initial use of Amaterasu charges chakra, a laspe to counter, it's instant afterwards.
Samehada already had issues handling a regular Katon, it absorbing Amaterasu and remaining in form is highly unlikely. But I guess I can't debunk that. Tobimara uses mark kunia which are chakra infused, something Itachi can see with his sharingan. Tobimamra planning to avoid Amaterasu by using ftg can be predicble, something Ay strategized against Minato. So, seeing as he doesn't have a similar fighting style as Minato, predicting his whereabouts would be a lot easier.
Those snakes have no feats to handle Manda, let alone 8 branch technique. GSB?!? Snakes (summonings) survive off chakra?!? That's a new topic I've never cisndered. Proof? Also, considering its prep, Manda dodges that effortlessly.
Duplicate clones is a good strategy. However, as we've seen, clones themselves are quite fragile most of the time w/ the exception of wood/Zetsu clones. With that being said, they'd have difficult bypassing Orochimaru fuuton techniques, which cover a massive range with considerable power. Itachi used Amatersau multiple times against Sasuke, 4-6 to be exact.
GSB is blocked with Orochimaru's summoned Gates. Itachi wouldn't need to use susnaoo.
SN: is this Zetsu Orochimaru? If so, they're more many ways for his team to capitalize
