You can buy bottles of liquor onboard but it will be held until the last morning and then you will have to claim it by Guest Services (unless they changed the pick up place). This was our experience onboard a few months ago. You can't buy hard liquor in port or onboard and bring it back to your room to consume.
DCL has a full menu of hard liquors you can buy by the glass for consumption onboard at the bars, lounges, pool, etc. but you can't buy hard liquor in ports or onboard for consumption during your cruise. It gets held and then returned to you the last morning at pick up. You get a claim check when you drop off purchased bottles and then hand that in when you go to pick up your booze. We bought booze in port and there was a table set up to drop off your booze right after you scanned your card to get back onboard.
You can bring on 2 bottles of wine OR a six pack of beer at embarkation and in each port to enjoy in your cabin (not in any public places onboard).
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