How do i know if i have a water infection with tender kidneys?

Water infection: If you mean a bladder infection, you have to have your urine cultured to see if there are bacteria there and get it treated with an appropriate antibiotic. If you have a fever and flank tenderness, you may have a pyelonephritis (P). A P is a more serious infection that will require hospitalization. See a physician to diagnose and treat your problem effectively. Good luck.