Just go in cosplay and wear a suit that reconstitutes your piss in to waterI remember in my younger days getting a large soft drink when I went to the theatres. I eventually downgraded to a medium because large was making me need to piss halfway through, but as of eight years ago, I just stopped getting a soft drink all together because I was starting to get up two or even three times during the movie to piss, or if I did stay through the whole thing, I was barely holding it in by the end.
Although there was one time I went to the theatre on a really hot day in the summer when I was still getting over a stomach flu and was still combatting dehydration I got a medium Coke anyway. The one positive about my situation was I didn't need to get once during the movie as a result.