It's not about beer having water, it's about alcohol decreasing the body's production of anti-diuretic hormone, which is used by the body to reabsorb water. With less anti-diuretic hormone available, your body loses more fluid than normal through increased urination.
(Part of that sentance is quoted from a result I found on a google search),
(Part of that sentance is quoted from a result I found on a google search),