Artificial "gills"
I've wished there was such a thing since I was a kid. SCUBA gear is so fucking heavy and unwieldy to lug around when you're above water, and you can't refill tanks just anywhere (and refill is pricy).
I can't wait until this is commercialized.
Of course it won't totally replace SCUBA or rebreather systems, because some water is anoxic (you can't pull oxygen out if it's not there), or toxic.