Prince Charles was speaking at a conference at Georgetown this week (taking a break from the royal wedding festivities, I suspect) to tell Americans that our fondness for dead cow is seriously straining the worlds water supplies. He's absolutely correct, that the amount of water needed to produce each pound of beef is almost outrageously disproportionate to the value of the nutrition produced. But I suspect that if you did a similar calculation of the amount of water needed to grow the grains to produce a fifth of gin or a pound of almonds it would look similarly disproportionate [I did a quick check online - 1) the numbers reported vary widely by source; 2) I didn't find any reported numbers for water used in the production of gin and the numbers I found for almonds were not useful]. Another thing is that probably 1,900 gallons of the 2,000 needed to produce the beef is used to irrigate the feed and if we didn't eat the beef we would still need to be fed so that 1,900 gallons of water would probably still be used to irrigate something.
If you look at trends for beef consumption and water use in the US for recent decades, it's clear that beef consumption is down and water consumption is flat. So maybe he's preaching to the choir in this country. What would really be helpful would be to convince people in the rapidly developing parts of the world that as they grow richer, greatly increasing their meat consumption is going to put enormous strains on their water supplies, which generally are pretty poorly managed at present. If they want to avoid significant negative environmental consequences of economic development it would be smart to manage water supplies in such a way that the majority of their water isn't used to grow food for animals that will be fed to people - especially if the people they are being fed to live in other countries.
Finally ... I don't really appreciate members of the one of the wealthiest families in the world telling me what I should eat. I'm a commoner, living in a constitutional democracy, that's just how I feel.