Snacks
Take snacks.
I’ve heard of relatively few instances where refreshments were provided at a candidate’s viva. Etiquette seems to suggest not eating in the viva; do at least take a bottle of water with you.
At the break or immediately after the viva you can feel drained from the effort, however pleasant your viva might be. Take food that might help. You might not want to eat in front of your examiners, but you might need to eat something in those intervals.
In short: don’t eat in the viva, but don’t go to the viva without taking food with you!