Prior to 2006, scuba diving was restricted in Greece. Nowadays, the regulation for diving is smoother, thus a large number of diving sites are open in order to be discovered!
Diving in the Greek islands is unique. There are lots of artifacts on the bottom of the sea as well as wrecks, caverns, reefs etc. The most popular islands for diving are: Crete, Mykonos, Kythnos, Corfu, Lesvos, Chios, Zakynthos or Zante, Naxos, Paros, Lefkas, Santorini and some areas outside Attica.
The life underwater is rich. You will find different species while diving. For the big stuff, you should dive around Galaxidi, the Gulf of Corinth where you can meet dolphins and turtles.
Here are some places you can enjoy diving in Greece:
Mykonos: Prasonissia, Peloponissos Wreck from 1930s, Dragonisi Island Caverns
Naxos: Baufighter Wreck (torpedo bomber from World War II), The Arado 196 German
Seaplane, The Dome.
Santorini: Nea Kameni, The Caves etc.
Crete: Messrschmitt Wreck, El Greco Cave, Deadalos etc.