I think it's quite easy to identify if in a given situation it is possible for an enterprising competitor to appear, or if there is some artificial distortion t

I think it's quite easy to identify if in a given situation it is possible for an enterprising competitor to appear, or if there is some artificial distortion that prevents such competition from appearing such as in your example. In your example, the real problem isn't that they're charging artificially high prices for water; it's that they're not allowing water to be brought in and then surprising attendees with high prices. If you want to fix that problem, fix it there. You could either require people to be allowed to bring water in (for obvious public health reasons, for example), or require the event organisers to prominently include the price of water in any advertising that includes the event ticket price, or require them to provide free water (again for public health reasons) and include the cost of supplying that in the ticket price.