some have none at all. The Greek Govt are very sensitive about diving as in
the past there has been 'disturbance' of archeological sites and keep a
close eye on the industry.

Some islands have quite a few operators i.e. Crete and some do not.

It is worth doing a search on the net.

For instance

http://www.dogsbreathdivers.com/Greece.htm

Diver1