I was told by some friends that just returned from a Western Carribbean cruise that they would each carry a bottle of water with them throughout the day in port. When you have drank all the water, go to a liquor store in town and you can purchase any clear liquor (i.e. vodka, rum, etc.) and pour it into the water bottle and walk right back on the ship with it.
As far as keeping the bottles of liquor packed in your suitcasees on the ship...I saw someone mention wrapping them in bubble wrap. I thought that was a great idea.
Hope this gives you some help.