All liquids are good for the kidneys to flush out any toxins. without liquids the kidneys can't work. however, liquids can be found in food, notably soups and stews. Drinking water boosts your liquids considerably as that's all they are; liquids. Drinking cordials means the sugar has to be broken down. amongst other things but no problem if you drink enough and don't already have a high sugar content in the bloodstream. If you don't like water, like me! drink plenty of weak drinks such as tea etc.
Sorry, I don't know what kidney pain can be like. I know when you get dehydrated you do get lower back pain but by drinking a few glasses of water over the next couple of hours relieves the ache.