A person asks the What If? fellow over at xkcd how much of the water on the earth today might previously have been in someone's soda.
The answer is a very small percentage. Although the water we drink is processed by our bodies and by sewage treatment facilities and then by nature itself, the best guess on how much water human beings have gulped down over the years of our existence is much smaller than the total amount of fresh water on Earth, let alone the amount when you add in the oceans.
Although, the answer guy notes, the long reign of the dinosaurs and their vast numbers mean that the water molecules in your glass right now, even though they are unlikely to have previously touched human lips, almost certainly once allowed T. rex or one of his relatives to quench their thirst.