Take a drink with you to the viva. After years and years of working with researchers, chatting to examiners and asking at institutions, I’ve found very few places where providing tea, coffee or water is part of the practice for the viva. Take something to stay hydrated.
Maybe take something to eat too. I don’t hear many stories of sharing cake or biscuits in the viva, but plenty of tales of candidates feeling drained as soon as the viva is finished. Take something you can eat to help energy levels in a break or when you’ve passed.
Little things can make a big difference.