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.