This blog posting makes the point very clearly that newspaper claims that beer is being sold by supermarkets for less than water are grossly misleading, as they are not comparing like with like. The comparison is between a very cheap bottom-end lager and a premium branded water. However, the average price of beer sold in supermarkets would still be far higher per fluid ounce than the price of water.
This is similar to the oft-repeated canard that pubs sell soft drinks for more than beer, whereas in reality, especially when you consider the actual quantities purchased, beer is on average considerably more expensive, drink for drink.