On every cruise I have been on (6 now - last one being OCT 2008) we would empty LITER bottles of water at each port and fill them with clear liquids, i.e. vodka/rum/etc, and walk back onto the ship.
The cost of liquor is so cheap on the islands, you would pound through the drinks/bottles before the next port just on principle.
I cannot guarantee that they have figured this trick out or not, but it's worth a shot (no pun intended).