I was there last Nov (and going again 2015 Nov). There was a big group of us, ranging from PADI Advanced divers, Instructors, right up to 200m+ Trimix CCR divers.
Everyone done the dives they wanted - everyone went and had a peak at the San Fran (50m+) and some of us went and dived the Nagano (at 60m)
Your certifications meaning nothing over there, if you are comfortable - all the dives are an absolute pleasure - most times you will end up doing at least 5-10mins of deco.
If you're not comfortable at the moment doing that - that I would suggest so Deco Procedures type course - at least you will know what you computer is beeping at
Every dive we did, they had a Alu80 hung at 5m - if you needed more gas
The wrecks (inside) are BIG and you can get lost if you go wondering on your own, most of the single divers followed a guide. So if you fancy a bit of exploring - perhaps a wreck course would be good (but a decent one - TDI, UTD etc)
Half the group was on Twins and half the group on singles, all on Nitrox/Air - no trimix (it was $1000 for a J !!).
I was diving a single 13ltr (100cu) and was getting 50+mins Runtimes - the water is warm, the vis is clear and there is no currents - its totally chilled diving.
I didn't feel the need for a twinset personally - otherwise I would of been down there a lot longer - we had to curtain some of our times due to lunch arrangements and people sat waiting for us topside getting cold..
The the two 'deep' dives of the San Fran and the Nagano I took a 5ltr of 50% to save some time on the deco.
2013-10-26 - Shinkoku Maru - 28m - 56min
2013-10-26 - ?? Maru - 36m - 58min
2013-10-26 - ?? Maru - 37m - 78min
2013-10-27 - Fumitsuki Maru - 37m - 49min
2013-10-27 - Kiyosumi Maru - 30m - 58min
2013-10-27 - Sankisan Maru - 30m - 51min
2013-10-27 - Futagami Maru - 26m - 48min
2013-10-28 - Hoki Maru - 42m - 43min
2013-10-28 - Yamagiri Maru - 30m - 60min
2013-10-28 - Unkai Maru - 37m - 53min
2013-10-28 - Kansho Maru - 31m - 48min
2013-10-29 - Momokawa Maru - 41m - 68min
2013-10-29 - Kansko Maru - 35m - 61min
2013-10-29 - Rio De Janeiro Maru - 29m - 51min
2013-10-29 - Daihatsu Junction - 10m - 47min
2013-10-30 - I-169 Submarine - 42m - 47min
2013-10-30 - Heian Maru - 33m - 60min
2013-10-30 - Gosei Maru - 22m - 56min
2013-10-31 - San Francisco Maru - 52m - 49min
2013-10-31 - Fujikawa Maru - 28m - 46min
2013-11-01 - Nagano Maru -60m - 71min
2013-11-01 - Shark Ville -21m - 45min
2013-11-01 - Shinkoku Maru - 32m - 51min
We have decided on a 'tec only trip' next time and take the rebreathers - time was too short most of them. But I had a blast even on a single - and I'm sure you will too!