Proof that Ay has more chakra than Kisame? Kisame was called the "tailless tailed beast" because he had chakra levels on par with bijuu...he fought the 4 tails jinchuuriki host yet Samehada didn't betray him. And feats are in Kisame's favor, like how he spit out a massive lake at 30% chakra.
He doesn't need to get close to the Raikage to absorb his chakra since he can do it from afar with Great Shark Bullet. If he uses his water dome first, then GSB, the Raikage can't swim fast enough to dodge.
Why wouldn't Kisame be able to use water dome? It only requires a single hand seal, not like the raikage is blitzing him before he does.