Two weeks ago we bought a fifth of rum and a fifth of vodka. One of the bottles went in to my snorkel bag with my fins & mask and the other one in Susan's (wrapped in our towels so they wouldn't break if dropped). They passed right through the x-ray machine with no problem at all.
If you bring booze back to the ship in the brightly colored bags from the duty free shops the cruise line will take it, write your cabin number on it and return it to you on the last night of the cruise.
Clear liquors such as gin, vodka, light rum etc could also be poured in a regular old plastic water bottle.
A few years ago a buddy of mine decided to put some whiskey in a plastic mouthwash bottle. Since plastic containers sometimes retain the flavor of what was originally in them that's a bad idea unless you like mint flavored whiskey :lol