If you are booked on a cruise log on to your account and chose fun shops then In-Room Beverages and it shows what liquor you can buy to have in your room but Rum is like 80 dollars a bottle.
Remember though, that the bottle you buy through the Fun Shops is a LITER - not a fifth. You'll get 22 pours from a liter as opposed to 17 from a fifth. Divide the $80 by 22 and you're only paying about $3.64 a drink - and no added 15% gratuity - BUT you've got to also buy your own mixer (or bring 12 cans onboard with you), and run back to your cabin every time you need another drink instead of just calling the drink boy over....depends on whether that's worth another $3 over going back to your cabin.