One other thing that's been mentioned in this thread that many people may not be aware of. If you purchase a drink while still docked in the United States, or in a territory of the United States (such as Puerto Rico), you will encounter the sales tax rate. Once you are officially in international waters, the sales tax won't be incurred, but you will incur the 15% automatic gratuity that is added to all beverage sales.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)