Mediterranean water isn't always cold and bad vis. I worked 3 months as a scuba instructor on Cyprus(the Greek Cypriot side). Early July the water temperature was about 28°C/82°F, by the middle of August it was 32°C/90°F, and then by the end of September it was down to 28°C/82°F again. It would however go down to about 24°C/75°F below the thermocline, around 20-24m(65-79ft) depth or so. Visibility was 30m/100ft on average, 20m/66ft on a bad day, and I had some dives where I could see easily 100m/330ft, if not more.Actually Greece is horrible for diving. The Greeks are xenophobic. They think that anyone who wants to go diving is planning to steal ancient Greece artifacts. You are presumed guilty until proven innocent, and they treat you accordingly. The Mediterranean water is cold and the visibility poor. I hear the cave diving is pretty good.
BTW, the diving in the Bahamas is rather poor. I'd go diving in Jupiter over the Bahamas, too.
As for the things to see... the aquatic life is a bit sparse compared to coral reef life. Not that many fish, octopus were rare, common stingray wasn't that rare, and there's morays. Other than the occasional crab the crustaceans were mostly just seen on night dives.
Biggest attitude problem with the Greek Cypriots? Many were very narcissistic/know-it-all, very full of themselves.