I used to work at our local blood bank and believe me I 'saw' just how important drinking water can be.  First of all.. being dehydrated makes all the difference in the world as to whether you can even 'find' a vein and if you do you more than likely won't get anything out of it.  I would show my donors how their blood would clot right there in the needle or in the bag and it looks disgusting.. excuse the graphics but it looks like snot and blood clots all in one.. and it was enough to scare them into drinking more water. The question I would put to them was .."Have you had anything to drink in the last 24hrs."? and they would answer "Oh yeah.. I had a large Iced Tea".. or coffee or soda it was always something with caffeine or even worse.. 'no' fluids at all.!! Our body is about 68% water if I'm not mistaken so yes not drinking water will affect you in so many ways.. it will cause headaches, dry skin, collapsed viens and lots of other things I can't think of right now.. and of course kidney problems and it affects all of our other organs.. think about it.. if you can die from thirst it's because our bodies need it.. and even though tea, coffee and soda are 'liquids' it's not the same. So just a little tip .. when ever you go to donate blood ..which I know everyone does.. or even to the lab for blood test.. do 'yourself' a favor and drink 'lots' of water or at least juice and you will more than likely save yourself from what can be a very 'unpleasant' experience..