hunting/fishing license lanyard - hangs around your neck ...
and is waterproof. This was suggested in the 'Practical Advice' section under 'cruise packing'. Sounds like a great idea and probably inexpensive from any hunting/fishing store.
Norwegian Sea, Roatan/Belize, Apr 2004
RCCL Splendour of the Seas, Western Caribbean (Cozumel/Costa Maya), May 2007
Princess Diamond, Mexican Riviera (Puerto Vallarta/Mazatlan/Cabo San Lucas), Sep 2006
Carnival Paradise, Catalina Island/Ensenada BC, Jun 2011