Can anyone tell me why most of the coral is dead on Okinawa, is it due to WW II?

no the coral is not dying b/c of WW II. The coral off the coast of Japan is being stressed in the same way as corals all over the world. Effects that are damaging the worlds coral reefs include higher water temperatures which cause bleaching, run off from agriculture which pollutes the coral, sedimentation from wave action and storms has smothered many corals, construction and dredging along the coast line has disturbed and destroyed a lot of reefs. There is a lot of work being done to conserve these reef coastal areas but developing countries and global warming are hard battles to fight on this planet.