Many papers show no effect of fluid intake on mortality. One very large study was run in the Netherlands in the 1980s, and published in the British Journal of Nutrition in 2010. Researchers followed over 120,000 individuals for a period of 10 years, and studied the relationships between fluid intake levels and mortality from heart attack and stroke. The authors found no link between total fluid intake or water intake and either cause of mortality.
Other studies echo this. This one found no impact of fluid intake (although, oddly, the authors exclude water) on chronic kidney disease or cardiovascular mortality. And this one, which randomized people into two groups — more and less water intake — and recorded outcomes like blood sodium levels, found no effect. And lest you think the reason to drink water is to improve your outer beauty, this study found no impacts of hydration on skin quality. (These studies all focus on healthy people. For individuals with particular medical conditions — kidney stones, for example — there are likely larger benefits of hydration.)
But some studies do find effects. Most notable is a study of 20,000 Seventh-Day Adventists, who were followed for six years to look for impact of fluid intake on mortality. The researchers found that drinking more water lowered the risk of dying. For women, the risk was lowered by having three or more cups per day, versus fewer than three. For men, three to four cups was much better than fewer than three, and five or more cups was a bit better than that. And at least one study has found that more than six cups of water a day lowers bladder cancer risk relative to less than one cup.