I have dived in several spots around Izu peninsula! There are plenty of dive shops around, and they can be really crowded.
If you are certified and comfortable diving with your own buddy, you can as well go to Izu by yourself. If you need a divemaster or an instructor, you probably better join a club from Tokyo.
Keep several things in your mind. Diving material (if you need to rent one) is very expensive, you can easily pay 5000-6000 yen per day only for the rental equipment. Then, you need to pay for the tank, as well as a fee to use the "facilities", typically around 2500 yen. And this is only for shore diving...
Another thing: diving spots can be really, really crowded, and it is not unusual to have to queue to be able to enter the water.
You can check this page with some information on diving in Izu :
http://www.sogival.com/plongee/show.php?&selcode=IZUE
For a diving club from Tokyo, check this site from Tokyo European Divers -
http://tokyoeuropeandivers.over-blog.com
sanji