I think Kisame will win
1. He should be able to see Zabuza in the kirigakure no jutsu. So, Zabuza's thick mist will be useless. If he cannot see Zabuza in the kirigakure no justu, he can merge with samehada so that he can locate Zabuza using his chakra detection.
2. In term of strength, Kisame is definitely stronger than Zabuza.
3. Both of them are quite a brainless (not much thinking while they're in battle). So, with point 1 and 2, Kisame is definitely the winner