Born to isolated world, Hahajima island has never got a chance to connected with continent.
Almost every time “isolated” means loneliness, but this time in Hahajima it means fewer human effect on environment.
In the sea you can see many fishes and animals which you have poor possibility in other place of Japan.
Fish flock? Yeah, it is so common to see that you will feel tired day by day.
One more thing, you can also see tunas, they stay in a fixed area. If you have the skill to control your breath and make bubbles from your mouth go along with the cliff, you can enjoy their swimming.
Here is a summary video.
Personally, I like sea cucumbers so much.
The most important is to have interesting and skilled divers with you.
When I in the ferry to Ogasawara, I saw a poster saying “You can’t come back in a week, meaning you have a week long holiday!”