Biomimicry designer and material scientist Jun Kamei at the Royal College of Art's answer is Amphibio – a sort of lightweight 3D-printed vest made out of a polymer that acts as both an artificial gill and a breathing reservoir that will allow people to hang out in the submerged megacities.
Designed in collaboration with the RCA-IIS Tokyo Design Lab, Amphibio is Kamei's solution to a projected future in 2100 when global warming has melted the ice caps and rising oceans have affected 30 percent of the world's population. The intuitive reaction to such a disaster might be to move inland, but Kamei believes that a better idea would be to adopt a semi-aquatic lifestyle using something like the Amphibio artificial gill.
Still in the concept stage, Amphibio is a biomimetic artificial gill that's based on diving spiders and insects that have a superhydrophobic skin surface that allows them to gather a bubble of air around their bodies. These bubbles act like gills that let in oxygen dissolved in the surrounding water and release carbon dioxide.