I carried on a small travel cooler, filled with pop cans and bottled water. I also had two other plastic bottles with alcohol inside. They didn't even look twice at the bag when it passed through the xray machine. They are looking for weapons etc not booze...but it does state that it is NOT allowed, so you do take your chances.
I also carried on a huge bottle of Kahlua, from Mazatlan, prepared to give it to them to hold until we disembarked, but they didn't ask for it.