For years I was allowed to bring a small mug into the theater as long as it was empty and clean. I never hid it. I would always show it to the door guard before entering and then keep it in my jacket pocket. One time (I don't remember the show or theater) I was not allowed to have it with me in the theater so I had to check it with someone and then get it back after the show.
My routine was to arrive at the theater maybe 45 minutes before showtime, drink a mug of coffee outside, clean and dry it before entering the building, and then at intermission (if I was thirsty) get some water from the drinking fountain and have a few swallows before act 2.
Beginning in 2011 (when I switched from hotels to airbnb) I usually stayed at a guy's apartment close to the theater, drank my coffee at his place, and then hustled over to the theater, arriving maybe 15 minutes before showtime.
I rarely (if ever) bought any food or drink at the theater with the exception of maybe a roll of candy. Are bottles of water no longer sold at the bar or was that practice discontinued years ago?