The ratio of water to sugar to sodium in sports drinks is done specifically to mimic the ratio found in blood to allow for rapid transit from the gut into the system. This helps the water, sugar and sodium circulate quickly and get to the muscles and brain where it is needed. Diluting sports drinks is a common practice, but it is not recommended. Sports and performance beverages are specially formulated to do several things - hydrate, provide energy in the form of carbs, replace electrolytes lost in sweat and do all of this with a fast rate of movement from the stomach into the blood. Diluting the drinks means they lose this advantage.
You also need to read the label. Sports drinks with 4-8% of energy from carbohydrates (or 15-16 g sugar per cup/250 mL) are the ones to look for. More sugar than this and you risk stomach upset during activity. Less that this and you risk running low on fuel.