I think it would be difficult to argue that someone who has purchased a ticket to the game isn’t a customer, irrespective of whether they make a purchase at the bar, as consideration has passed from them to the club to enter the licensed area. That surely makes them a customer.
Potable water not being available at a servery that dispenses drinks, including draught beer, can’t by any objective analysis be held to be reasonable. It would be a preposterous defence to run, especially given the resources of the club.