Greece is in southern Europe, bordered by the Aegean, the Ionian and the Mediterranean Seas. It's the only country in the EU that doesn’t have any border with the other members.  Here's the map.

Greece is not really known for its scuba diving. Until recently, the government warily restricted diving due to it's paranoia of ancient artifacts and important archaelogical items being removed from underwater sites. Thankfully, the Greek government has loosened many of its regulations on scuba diving, although it is still restrictive for specialty dives like night and wreck dives.

Because there hasn't historically been much recreational diving, much of Greece's underwater expanse is largely unexplored.  This is quite exciting because there are dozens of islands scattered around Greece in the Ionian and Aegean seas with hidden gems yet to be discovered. 

Diving opportunities are everywhere but probably Greece's most famous dive site is that of the HMHS Britannic (sister ship to the Titanic) - lying in about 400 ft (120 m) of water and first discovered and explored by Jacques Cousteau in 1975.  However, due to the depth of this dive, its for technical divers only.  Still, for those of you who aren't, here's the YouTube Video.

• Corfu:  The Nisopoulo wreck which is either a Greek or Roman wreck that is surrounded with amphora.  Also, in the same area are numerous good quality reefs. 
• Crete: The Messerschmitt ME109 aircraft which lies to the west of Agia Pelagia at a depth of 24 metres is worth diving.
• Zakynthos:  Marathonissi (Turtle Island) off the Keri Peninsula is probably one of the best dives, where if you're lucky, you will see giant loggerhead turtles, plus octopus, moray eels, cuttlefish, flying fish, grouper, bream, wrasse and parrotfish.

• A must-dive destination is the island of Santorini which attracts many divers eager to hover above the abyss at one of the most famous volcanoes in the world. 

• There ae heavy restrictions on where you can dive. The Greek government is attempting to protect unsalvaged ancient sculptures, walls, and other artifacts by limiting scuba diving to a few approved areas.  However, as scuba diving grows in popularity, it seems that some of the restrictions are "unofficially" easing.  See the Underwater Archaeology link below.
• Due to over fishing, there are not many large fish, although plenty of starfish, octopus and groupers.
• Spring and Autumn are the best times to visit, the weather has warmed up and the crowds have not yet arrived. However, diving is still relatively undeveloped in the country and you are unlikely to find diving groups fighting for prime sites at any time of year.