Like Shooting Fish in a Barrel
This expression brings up so many questions, I don’t quite know where to start.
First, exactly who is in the barrel? You, the fish or both? If you are in the barrel, shooting anything is awkward although scores points for entertainment value. If the fish are in the barrel, why are they there, and if you have them contained in a barrel, why do you need to shoot them? I don’t think I need to point out if you are both in the barrel why shooting them is ill-advised.
Secondly, I know this expression is supposed to mean something is easy. But, I’ve watched CSI, and that qualifies me as a ballistics expert. Water stops bullets. Mythbusters reports that you only have to be 3 feet underwater to be safe from a bullet fired at a 30 degree angle.
And finally shooting fish, if you could hit one, is messy and not the preferred method for a nice fillet. I’ve never seen it done on “Chopped,” so I know it’s not sanctioned by chefs or probably the ASPCA, for that matter.
So, I propose that “like shooting fish in a barrel” actually means something so stupid you would never actually do.
Snooty Call 
I agree with you on this one. I’ve never quite understood this saying.
As for Mythbusters, they actually have tested this adage: http://mythbustersresults.com/episode91
I would suggest replacing this cliche with something else, but what? “As easy as winning a one-man election”?
I know. I did google and find the Mythbusters report on this subject. However, they concluded it was easy to shoot a fish in a barrel, albeit a dead one. And since they didn’t agree with me, I decided not to cite them. As a blogger, I do not find myself constrained by any journalistic integrity, whatsoever.