What is the relationship between fitness and water intake?

Water = health. In populations where water intake is low, increasing water improves weight loss and BMI, thus lowering chances of type two diabetes, hypertension, the primary causes of lifestyle disease. In healthy people who don't drink sodas and sugary drinks, drinking enough water is essential for good health.
Not sure exactly. What you mean. But the more active you are the more your daily water intake needs will likely be. And up to a certain point(as too much of anything isn't good for you), the more hydrated you are the better your fitness performances will be.