Hello, have a looking at Amami Oshima, just north of Okinawa, and much smaller so easier to get about, Divers Cove Amami is a English run dive shop that offers boat and beach diving, plus can they arrange your accommodation and provide free airport transfers. Diving in Amami is awesome lots of fish life and lots of coral the dive sites are not over dived as Amami gets a fraction of tourists that the bigger island gets. JAL flys to Amami from most airports (including from Okinawa) within Japan and Skymark airlines offers competitive budget flights from Kagoshima to Amami, check out scubajapan.com for more info.
Amami's whale season is between February to April, turtles come ashore to lay eggs around June and they hatch about 2 months later and dolphins are buzzing around most of the time!