Japan is a very large country when it comes to diving as it is chain of islands that runs North-South. So we have ice diving in the north and subtropical areas in the south.
Okinawa is the southern most prefecture and stretches from the main island of Okinawa to Yonaguni in the southwest.
Okinawa diving is year round with the different seasons bringing different critters.
Jan-Mar is humpback whale season in the main island and Kerama islands with the giants of the sea coming in for calving and mating. This allows for numerous sightings and even chances to snorkel with these guys. (Water temp 20-22C)
Also in Jan - April we have the schooling hammerheads in Yonaguni- huge schools come visit this tiny islands that is actually closer to Taiwan than any place in Japan (Water temp 24-26C)
Starting in April water temps will rise to 29-31C by June and diving in Okinawa concentrates on colorful reefs and crystal clear water with lots of variety of sea life.
The main islands of Okinawa are: Okinawa, Miyako, Ishigaki,Iriomote, and Yonaguni
There are also numerous smaller islands with great diving: the Kerama Island group, Tonaki, Kume, Aguni, Ie, Iheya, Hateruma, and the numerous small islands around Ishigaki/Iriomote area.
As you travel north of Okinawa Yoron and Amami island both have outstanding diving year round.