Ask HN: What "futuristic" technologies have not yet been realized?
Obvious ones that come to mind: teleportation, flying cars, true virtual reality. Any more?
We've had SCUBA for a while, actually. :)
But I assume you mean something that pulls oxygen directly from the water, like gills?
(Not to mention the catastrophic nuclear waste accident that sends the moon out of its orbit and into the deepest reaches of space - that one's overdue by 11 years now).
(sorry)