It is probably just as easy to get to Guam or Saipan as it is to get to Okinawa and also probably cheaper (unless you. The added bonus is 85 deg F (29 deg C) if it is warmer water that you are looking for.
I can't agree about travel times. From Central Tokyo, Guam is probably 8 hours travel time, Okinawa is around 4. The whole experience is a lot more pleasant too! You can be diving the same day you fly if you head to Naha.
Guam: 90 mins to Narita, 2 hours check in & immigration for interntional flight then 4 hours to Guam and queuing up again for US immigration.
Okinawa: 30 mins travel to Haneda, 10 mins check in (it actually took me 2 mins), 2.5 hour flight.
For JAL internal flights at Haneda, you can use a barcode they send to your mobile phone to check in. I got off the monorail, went up 3 escalators (5 mins walk), used my mobile to get through the 2 minute security check and voila - arrived directly at the gate where I used my mobile again to get on the plane. From getting off the monorail to being at the gate took less than 10 minutes.
Compared with the joys of Narita, and the diving being better in Okinawa there is no way I'm ever going to Guam again, except as a stop for Palau/Yap/Truk etc.
Skymark fly to Naha for 12k for a single leg, which is pretty reasonable.
Geoff... how's the diving in Shimoda? Not been to that part of Izu yet.
It's nice, Mikimoto is a great site... huge currents, reasonable chance of seeing hammerheads, superb soft corals and lots of marine life. Definitely the toughest diving I have seen in Japan, definitely not for the inexperienced - I think the local operators put a minimum 50 dives experience limit on it.
