How important is good hydration for weight loss?

Improves weight loss. Study showed - after three months the group that drank water before meals had lost about 7kg (15½lb) each, while those in the thirsty group lost only 5kg.
Somewhat important. Good hydration (drinking water each day, instead of coffee, juice, or sodas) is good for all persons. For a person trying to lose weight, drinking water when hungry will cut down on some of the hunger, so the person will be less likely to eat a large amount at a snack or meal.