From my Observations
It depends on the ship, the port and the time of the year.
In San Juan, there is a duty free shop inside the terminal and steps from entering the ship. The powers that be at this port allow you to purchase all you want, stick it in your carry one and walk on the ship -- no questions asked. In the island ports like St. Thomas, you can purchase the alcohol and many times, walk right on the ship with it and no one stops you. I have done this in St. Thomas, Barbados and Cozumel.
In mainland embarkation ports, I would suggest you pack it in your checked luggage. They do not, I repeat DO NOT open your luggage without you being present. They are handling upwards of over 6,000 pieces of luggage and are not going to bother with one litre of alcohol. However, if you check a piece of luggage that weighs 80 pounds and upon xray examination shows several full sized bottles of alcohol, you will not receive that bag along with the others, you will instead receive a message to come down and open your bag for inspection. This is only done when the amount of alcohol is excessive (more than one, maybe two bottle is my guess).
Now, all of the above goes out the window during spring break time. As we all know, the young but legally able to drink crowd still believe pouring liquor down their throats until they are "falling off the ship" drunk is a great way to enjoy the evening. The cruiselines have to be able to control the amount of liquid problematic passengers can consume. If, after a day or two, any particular passenger(s) are shown to be alcohol-challenged in their demeanor or behavior, the cruiseline can code their sail and sign cards to limit or ban their purchase of alcoholic beverages.
Therefore, during spring break and senior trip times of the year, the cruiselines will become more vigilant. Have you ever seen a young college (or younger) girl falling down drunk on these ships. Sexual assault (or worse) waiting to happen. The cruiselines know that even if they aren't responsible, they will be sued so they have to have some level of control.
At other times of the year, they will usually overlook a small amount. However, if you need 2-3 full-sized bottles of alcohol to bring with you, perhaps cruising is not the most cost effective vacation for you. I would suggest an all inclusive resort.
(cruzin' solo by design)
Freedom ('09), Imagination ('08), Victory ('08), Destiny ('08), RCCL Navigator ('08), RCCL Majesty ('08), Glory ('07), Liberty ('07), Fantasy ('07), Destiny ('06), NCL PofAm ('05), Fascination ('04), Victory ('04), Fascination ('04), RCCL Voyager ('03), Fantasy ('99)