This location is very close to Churaumi Aquarium (just slightly to the north of that bay on the coast) - Churaumi is massive. They have whale sharks, rays etc. Its either the worlds largest or 2nd largest aquarium - so these nets could well be something to do with that, breeding grounds etc.
Its just a guess, but I don't this this specific type of netting anywhere else around okinawa.
Farms - Aquaculture 
Its just a guess, but I don't this this specific type of netting anywhere else around okinawa.