It sounds to me from what the guy says that the comic sizzling up all the water was an unexpected side-effect, so I kinda doubt that they were doing it in advance. And whether or not Wondy is immortal is a much-argued point; some sources have it that she lost her immortality when she left Themyscira. Green Lantern could MAYBE have done it - but the guy clearly says when at the museum that 'this is the last water on Earth', so the clear implication is that he didn't.