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.
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.