Yonaguni:
If you've never dived with Japanese ops before, they are into the group orgy thing. Where everyone goes to around as a group and are herded like sheep. If you want to do gaijin wandering around on your own stuff, just talk to your guide before hand. Be prepared to see a lot of diver contact with reef if you have many beginner japanese divers.
They usually have a set route to do the monuments, going down to do the "river channel" and then up to the slabs with "writing" then move on up to the monument steps before ending at the big ball on a slab which many japanese like to do a little underwater prayer. Most of the dives are drift dives.
Do try the hanasake that's locally brewed... it's 60% alcohol only
I had my first fresh marlin heart sushi washed down with the hanasake. That was harsh!
Who are you diving with in Yonaguni? You'd better brush up on your japanese if you're not diving with an ops with an english speaking DM. There's a great japanese curry place in Kubura. Accomodations are very basic. Tatami mattresses take a bit of getting use to.
You're a little early for the hammerhead schools though. Have fun. I had a great blast in Yonaguni and I do want to return to do the caverns again. We had just started diving that site in 99.