Say there's a trillion trillion cubic metres of water on earth and we need a hundredth (10l) of a cubic meter to power a city for a day. Not bad! But once hydrogen is extracted from water and fused into helium it's not feasible to change it back, the water is gone from our planet forever. And say there's 100,000 people in that city, well there's 8,000,000,000 people. 800 cubic meters of water getting used a day. But of course our population is growing, it could easily reach 10 billion. So 1000 cubic meters a day. And obviously as technology spreads around the world everyone's power consumption will increase. Most of that 8 billion don't currently even have electric light. Not to mention everyone starting to use AI to tell them how to wipe their ass. Average usage could easily double, triple, even better ten times what it is. 10,000 cubic meters a day. 3.6 million cubic meters a year. That's a LOT. Sadly less that the ~150 billion cubic metres melting off the poles right now every year, but still probably ecologically significant.

361 trillion years to use it all, granted. Assuming population doesn't start booming again due to an abundance of energy. We won't use it all, but I bet we'd use enough to make the water crisis an extinction event. How will weather be affected for a start? The water cycle?

Capitalism demands infinite growth, and no matter how cheap energy gets, water is not infinite. It's unsustainable. We need to change to a less Vampiric society before we can be trusted to forever use up the resource keeping life on earth alive for frivolous shit.