How does that work in a market where you are bidding amongst other participants and capacity is constrained? Supply is limited and demand is growing, the market response is to charge more.
Well, just off the top of my head... if you wanted to treat the increased prices caused by the spike in demand as an externality (and I agree that's not exactly the right way to think of it, but people seem determined to in this case and I can't really fault them for that given the practical effects of such price increases) I think the way to do that would be to give existing consumers futures contracts for water for the next ~10 years proportional to their existing usage.
People could then either use the futures contracts to keep the price they pay for water the same as it was, or voluntarily reduce their water usage and sell the contracts to the datacenter for a profit. Same deal for power.
This would of course be a bit complicated for most people to manage, so maybe instead of giving consumers the contracts directly you could have some intermediary manage the contracts for them with approximately the same result.
