Dehydrated vs. Dry
Dry and dehydrated skin aren't the same thing. Dry skin lacks oils to hold in moisture, causing cracked skin that can even bleed. Or your skin may have lost moisture because of low humidity. Dehydrated skin doesn't have enough moisture on the inside.
How Lack of Water Affects Skin
The layers of your skin need moisture to work the way they're supposed to. Without enough water, skin can't stretch or protect you as well as it should.
You may notice your skin has a flat look to it. Dull skin doesn't have enough moisture to give it the radiance and shine healthy skin has.
You may notice darkness under your eyes. This can happen because without enough hydration, the skin around your eyes can pull away from your eye sockets and cause a sunken look.
Wrinkles are a natural part of aging, sun exposure, and genetics -- and no amount of water will prevent them. However, very fine lines may be a result of dehydration. More moisture can make skin plumper, so faint lines fade.
Both dehydrated and dry skin can cause itching. Skin that cracks from dehydration can feel itchy. It can also let in bacteria.
Bags Under Eyes
As your skin loses its elasticity (stretch) from a lack of moisture, the skin under your eyes can "pool" and cause under eye bags.
To tell if you're dehydrated, you can do the "pinch test." Pinch a bit of skin on your arm between your thumb and forefinger. If you're well hydrated, it should go back to its original position a couple of seconds after you let go. If it doesn't, time to throw back some water.