Bladder cancer has two primary risk factors—smoking and environmental (usually occupational) exposure to toxins (such as dry-cleaning solvents, pesticides, and paints). Because men are more likely to smoke and be exposed to these toxins, they have highest rates of bladder cancer. To reduce the risk of developing bladder cancer, many have wondered if increased water intake might dilute the concentration of toxins in the bladder and reduce their time in contact with bladder cells. However, there are conflicting results from studies looking for a link between bladder cancer and water intake, largely because they were observational studies that examined different criteria. The only reasonable conclusion is that we should drink enough water to avoid dehydration.
P.S. Water needs vary from person to person, based on activity levels, age, and other factors.