I was going to suggest the same thing!! They are better known as sports casts. I know as a matter of fact they exsist and that they work. Medical1 suggests maybe cutting it off and wrapping, elevating and ect...There is also always the fall back plan!!wetvet once bubbled...
Consider asking your doc for a fibreglass cast. They are stronger, lighter, and waterproof. We use em on dogs, cause they never listen when you tell em not to get their cast wet. I would get it cut off when you get home, and replaced, because the cast padding will be all compressed and bunched. Bouyancy of fibreglass could be an issue too......ankle weight may be helpful.
Good luck.
Wetvet
Damn I broke my ankle (I fratured my back 2 weeks before a non-refundable trip to Cozumel) I guess I have got to call this a shore visit for a change...Though I am told what you want to do is possible in many ways...I would suggest highly to get the cast changed to the sports cast a few days prior to departure.
Good Luck!! No matter what you do be sure at least one person who cares knows that you are somewhat compormised and may need some assistance..Just so you have someone prepared to help you in case of need...
Good Luck, Hope it works out for you