Eating chocolate or oily foods causes oily skin and acne
Sebum causes acne, which is made and secreted by the skin. Dairy products and high glycemic foods can trigger acne but they don't cause acne.
Drinking water will fix dry skin
Use high-quality daily moisturizers to improve skin dryness, such as glycerin, hyaluronic acid, ceramides, urea. Water helps with overall body dehydration but it isn't to help skin dryness.