- The Chakra that Kisame stole from Killerbee really only restored Kisame to his former strength.
- Kisame left his sword behind to drain Killerbee. As one of the Seven Swordsmen, Kisame really does rely on his blade.
- While one could argue that Samehada wouldn't make all that big a difference against Guy (who doesn't rely on chakra-based attacks), the sword itself is critical for Kisame's fighting style. Without it, he cannot block or parry close-range attacks, cannot slice/drain his opponents (which would still be harmful to a close-range fighter like Guy), and cannot replenish his strength (which may have helped him recover from Guy's Afternoon Tiger ability).
The fact is, Kisame was fighting primarily to escape and deliver his message to Madara. He may have been surrounded by water, but it's not as if he can't just effortlessly summon as much water as he could need. The presence of the ocean merely saved him the trouble.
Guy's Afternoon Tiger, while momentarily allowing him to take down Kisame, actually did more damage to Guy's body that it did to Kisame's. To me, that speaks volumes about the difference in their power. Kisame could have kept fighting after a momentary breather (where he not surrounded by Yamato/Naruto/Killerbee), but Guy could barely move. However, while Guy's power-up lasted, he DID have complete power over the momentarily stunned Kisame.
I'm just providing a different view of their fight. While Guy did win, the exact factors that contributed to his win against Kisame are, in my opinion, up for debate.