There were many different strengths of beer which (often)corresponded to different runnings/washing’s of the malt. The initial wort has higher amounts of sugars/fermentables while the later washes have significantly less. It could get down to 2-3% which. As to being a diuretic I think people are confused about how that works. It’s not like adding minus water if you drink coffee or alcohol, eg you wouldn’t die of dehydration if all you had to drink was coffee or beer.