I wish I could find the link now, but there was a very interesting (and, based on my admittedly rudimentary knowledge of history, high quality) thread on reddit's /r/askHistorians that dispelled this 'myth'.

It may be diuretic but, for obvious reasons, no alcoholic drink is entirely made of ethanol. Just like sugared soda might not be the most healthy choice of hydration, it's still gonna keep you hydrated.