A good presentation is hard work. Spend time researching, writing, and honing the presentation. People who fly from place to place giving presentations or who give presentations often should continually replenish fluids. Water is best, juice is next best. Alcohol and caffeine cause further dehydration, and should be offset by even more water.
Water is useful during the presentation, to preserve the voice.
Bibliography Lowman, Joseph. Mastering the Techniques of Teaching. The Visual Display of Quantitative Information. March 9, 2000.