If Kisame throws water and Gaara throws sand,they mixes(no effect).It's just as 2nd mizukage did.Using water against Gaara is useful to decrease the sand structure resistance,but not to surpass it. He would need physical attacks(maybe Samehada) after throwing water, and I think that's too hard for such a slow character as Kisame.And obviously,Gaara wouldn't be still all this time.Gaara is pretty slow and has no counter to water so kisames water clone could probably hold him in a water sphere like it did vs gai's team.