Duty free goods on cruise ships are legal because they are not sold to the people who live within the destination being visited. Also many cruise ships will close their shops whilst the ship is in port to abide by the local custom laws.
If you are bringing in alcohol, you are allowed only one liter of alcohol per person under the personal exemption rules. You can bring in 200 cigarettes (or 100 cigars) per person under the personal exemption. Beyond that, you will be charged duty taxes on those items — even though the value is still less than $800.
Standard alcohol packages average $55 to $65 per person, per day, and the price likewise varies both by cruise line and drink inclusions.