We packed wine bottles in the luggage.
Also used large plastic flasks for hard liquor.
Did it many times....never broke a wine bottle and the cruise line never knew.....as far as I know.
Also another trick when you hit a port is take a large water bottle in your backpack, buy some vodka and fill ‘er up....any search getting back on the boat it looks like a bottle of water.
I brought the wine bottles, too.
On my last 2 week cruise I did the unlimited booze deal @ 1/2 price. Oh lordy, never again.... I don’t drink anywhere near that much.
Great idea with the water bottle.