@Fred: This isn't my impression of the lesson learnt for most of us though. However, I do believe that some people's capacity did grow, and eventually everybody simply got used to the drinking and peeing routine.
@Lee @Brian: Unfortunately I don't have much to say about ceremonial events. We do have parades, such as the basic training graduation parade, but often these aren't terribly long, and our weather makes it such that much of the fluids get sweated out when under the hot sun. When training outdoors, heat injuries typically present a far greater concern than full bladders.