@Enzeru:
That was the case only once because Luffy was underwater so he had to take his time for preparing the attack. As was seen in one of the recent chapters it took him a mere seconds to prepare and use Elephant Gun.
I don't think that is the case….
do you mean the Elephant gun against monster Chopper?
First, Chopper didn't control the body, it was the mindless monstrosity back from Enies Lobby, the one who doesn't dodge and simply takes everything in. Plenty of time to prepare. As for water weakening the attack, agree. But going into gear third always took time, and the only difference between Elephant gun and a normal g3 attack is haki.
But more important, Elephant gun needs a drawback, else it would be the to-go attack in every situation. Just as TheCrystalShip said, it was always a "problem" with g3 - but thats okay since he has plenty of other moves at his disposal, whatever the situation calls for.