I can't really see how they'd get themselves into a right can of worms, uness you are suggesting either;
1. They're doing it just to cause you trouble and that means they might take it too far and start spiking drinks, or,
2. You could spike your own drink afterwarRAB and blame it on them, getting Cineworld into BIG trouble.
Beyond that, I can't really see how spiking drinks would come in to it.
As far as taking water in, of course they can stop you. They can deny entry for no reason at all, so long as you haven't already paid.
However, if you are an unlimited card holder, I think they'd have a hard time holding it up in court. Taking in a coke is one thing - as they can claim you are unfairly using their facilities and affecting their business - but a bottle of water is likely to be laughed out the courtroom. Fresh water might not be a right under the law, but most consider it an essential, as opposed to a luxury item.