My skin is dry because I do not drink enough water.
Not really, no...
Unfortunately, it's not that simple. While drinking water is essential to our health, there is no study to suggest that consuming more water would have a direct impact on skin moisture. In fact, the skin dries when its barrier is not able to conserve enough water in the stratum corneum (a superficial layer of the skin) and evaporates into the atmosphere. Specifically, if the skin contains less than 10% water, it will tend to become dry. Several factors may exacerbate this problem, including, but not limited to, unprotected exposure to the sun, season, temperature fluctuations, health status, environment, type of home heating, and use of improper cosmetics .
Whatever the source of the problem, and there may be more than one, it is by restoring the barrier of hydration of the skin that it can be allowed to remain healthy. The best tools to do this are to use proper cosmetics and wear sunscreen, minimum SPF of 30, every day!
Draelos ZD. Therapeutic moisturizers. Dermatol Clin. 2000; 18: 597.
Wildnauer RH, Bothwell JW, Douglass AB. Stratum corneum biomechanical properties. I. Influence of relative humidity on normal and extracted human stratum corneum. J Invest Dermatol. 1957: 56: 72.