I live in Ohio (our liquor must also be purchased at a state store) and the cost of the alcohol onboard is about half of what I pay at home. If you are doing a western Caribbean cruise, the duty-free allowance on alcohol is 1 liter per person. If you purchase your alcohol in St. Thomas (eastern Caribbean) the duty-free allowance is greater. You can always pay the extra duty charges if you bring back more than allowed, which for me is still a substantial savings. I know many people purchase 4 liters packaged in heavy-duty carry boxes and are just waved through customs without a glance, but you should be prepared to declare it.