Is drinking a cup of water before breakfast good for health?

Water: Yes. Drinking water regularly will both control appetite and also help you remain well hydrated. Would encourage you to drink 8 oz of water several times a day.

Yes: Being well hydrated at all times is advisable unless there is a specific reason to be restricting fluid intake. And, of course, even water can be toxic if taken to excess. You need to replace what you evaporate plus enough to keep your urine flowing but not so much that you overwhelm your kidney's ability to regulate electrolyte concentrations.

Yes...: There is evidence that drinking 2 cups of water (500 mL) just prior to eating a meal causes people to eat less (by 75-90 calories). This is good news If you need to lose weight for health. Also, one theory of prolonged life is calorie restriction--fewer calories in animal studies showed longer life span. You do need to maintain a healthy weight and balanced diet, though. So drink up!