'One' does not need to drink 2 litres of water every day. We all buy into these health hypes without question don't we? All liquids count even coffee, unless perhaps you only drink double expressos!
Caffeine is not a big deal, and drinking decaffeinated brings its own problems.
Yes, we should drink what is adequate for our needs, and dehydration does indeed cause fatigue as rosario said, but it's liquids not just water, though common sense should tell us fizzy drinks, or indeed any sugar laden drinks (and this includes fruit juice) are not good for us. Milk, tea, soups and stews, fruit and vegetables all contain water.
One cause of UTIs is not emptying the bladder completely, did your urologist mention that mazza?