History

You're both right depending on the region and time. Weak Beer being used when water wasnt necessarily safe was a thing, it was also a way of getting a second run out of your grain after brewing stronger beer. Small Beer as the weaker sruff was called was still around 3% which is at least adequate to make it spoil a lot slower than still water. That being said your Beer for getting drunk was brewed pretty strong. This is all based mostly on medieval western Europe. Beer is older than recorded history so there is a lot of times and places where for your mileage to vary but it does seem like weaker second run beers were used when the water sucks but also cause it was a flavored beverage.