The streets are knee-deep in water, which led to humans wearing swimsuits all the time. We can deduce that this happened because global warming led to the water levels rising to a level where humans were forced to adapt. I don't think it's a reach for me to say that the message of this manga is that if we don't want to live in a world where girls walk around in skintight swimsuits receiving mock facials and almost exposing their breasts, then we need to do something to stop global warming! And who would want that kind of world? It sounds horrible.