I think they just got along, they wasn't "friends", but they never had any problems on getting along. Everytime Kisame would talk about rumors of Itachi's past he will just stay shut, so it showed that they wasn't friends, but they never got into an arguement, or never got to the point where they wouldn't get each other's back. For example, when Kisame acted as a "bodyguard" in order to let Itachi and Sasuke fight alone, without no interuptions from Sasuke's team. They somehow understood each other's pride and never had problems.