Well, what's evil is the ridiculous price of spirits on a cruise liner :P Here's a trick to re-cap a mouthwash bottle for your next trip through customs and/or through the porters.
Step 1: Aquiring a Sealed Cap
So you've finished you're bottle of mouthwash? Well, you're going to need another one - so go grab that one.
Gently remove the cap(squeeze the sides and twist) from the new bottle without breaking the perforated seal. Replace that cap with your old one.
Step 2: Wash and Fill Empty Bottle
Give the bottle a good washing, but take care not to damage the label. Next, fill with your spirit of choice - just don't try a carbonated beverage (unless it's carbonated at 1 atmosphere).
Over fill = good.
Under fill = suspicious.
Keep that in mind ;)
Step 3: Re-Cap
Take your "sealed" cap and place it on your newly filled bottle. Ours went on easily and screwed in just like it came from the store.
Pack in your suitcase just like you normally would your mouthwash.
For those of you attempting with a container that's not opaque (this mouthwash contains hydrogen peroxide - which is photosensitive), add a few drops of food coloring ;) It won't change the flavor, but will become a very potent stain adder. So be careful ;)
Step 4: Implications
I can only imagine. Without being able to cite, I'm going to go out on a limb and call this illegal - or close to it. The cruise liner set the rule - not customs (to me knowledge). But there may be duties on alcohol when you enter other countries.
Similarly, this could be used for other forms of "evil." I hope you're not too paranoid to think that the cutting edge of this evil is college spring break at sea ;)