Avoiding dizziness before, during, and after assisting on a surgery can be difficult. I've had trouble getting enough water in so I made it a habit to drink 700 mL to 1 liter of water at least 1.5 hours before scrubbing in. I found it really useful to bring Gatorade powder to mix in with water since it makes me feel a lot better than just water alone.
Self-care on a surgical service is hard, but hydrating well is the key. I learned that 4-5 gulps of water was enough to help me not feel dizzy while guaranteeing a manageable bladder while 11-12 gulps would restore my volume but require me to go to the bathroom before the next case. It sounds crazy, but it made my surgery rotation so much more bearable. Surgery is a rotation where things run on a tight schedule and you need to be in good physical condition to last on 2+ hour surgeries on little sleep and few chances to eat. Always carry snacks and stay healthy!