...because the shape of the water bottle isn't uniform either. Part of my new year's resolution is to drink 4 liters of water per day, so I spend a lot of time looking at those lines. Edit: 4 liters is a lot, but appropriate for my size and activity level. Without conscious consideration I don't drink enough water. It looks blue because the bottle is blue.
As I understand it, your bladder collects urine, which is water and waste products. Your body, your kidneys, etc, are always trying to maintain a Goldilocks level of hydration. If you drink a lot of water, your body removes the water from your bloodstream and puts it in the bladder so you can pee it out. That way you don't get overhydrated. If you're underhydrating, you will still have to pee eventually to void the water soluble waste products in urine, like uric acid. And if you don't replenish this you'll get dehydrated.
As I understand it, alcohol dehydrates you -- it removes water from your bloodstream and into your bladder, which gets full and causes you to pee. Your electrolytes get more concentrated, and your muscles are less effective. Your body doesn't have the water it needs to properly function unless you drink more water.
Caffeine on the other hand does not dehydrate you this way. What it does is irritate your bladder, causing you to void whatever was in there before you would otherwise need to, but not affecting your electrolyte balance in your blood.
So while both are "diuretics" in the sense that they make you pee, only one is dehydrating you, and is a "diuretic" in that sense.