Kefalonia is really nice and the Ionian sea has crystal clear waters. Dont expect to see the same marine life as in Red Sea but there is still enough to see.
I cant comment on any dive centers since i havent used one of them but i think its worth to dive
a) The Mellisani cave
b) HMS Perseus a WWII British submarine (its at 50m though you can still see much of it at around 40m)
The Great Escape 
Enjoy your dives
Photos and videos are not mine. There are more if you want though.