I haven't been to Athens yet, but I've done multiple dives at CSSP and a couple at Blue Lagoon, which I will be diving this weekend while my wife completes her OW dives.
CSSP is definitely not what I'd consider "clear". When I did my OW dives there, vis was limited to about 3'. Granted, this was in November '14, but it was a bit of an odd feeling doing my first OW dives in water I could barely see my buddy in. Max vis you can really expect out there is about 15'. My wife tried to do her OW dives out there this past May and called it after the first 10'. She absolutely HATED the vis. And it does get cold. Last time I dove out there (April) we dropped down to the plane on the far end of the lake, which sits around 38'-40' and bottom temps were right around 54 degrees. Full 5mm suit with gloves and hood and I was completely comfortable.
Blue Lagoon offers much better visibility. First time I dove there was mid-May and vis was 25'+. Downside is there is no marine life and max depth is around 30' in lagoon # 1. Really nice facilities and they cater to divers. Just make sure you make a reservation a week or so in advance. I did dive there about 3-4 weeks ago and vis was limited to about 10'-12' feet, but it was still a decent blueish-green color. Bottom temp at 23', where the platforms are (Lagoon 2) was a very warm 89 degrees! So...plan on not wearing an exposure suit heavier than a dive skin. I usually dive a swimsuit and UnderArmor compression top. Entry fee for divers is $17/day and swimmers are only allowed to be there from 12-6. I believe divers can camp for an additional fee, but I have yet to do so. Would be a cool place to do night dives, though.
If you make a reservation, Lagoon 2 is the one centered around divers. Swimmers are at Lagoon 1. I've had shelter 7 at Lagoon 2 twice and it's a few short steps away from a great walk-in spot. Flash Point is a good one, too, but it's more of a giant stride, which they pretty much discourage. Most of the larger groups/classes take the shelter on the very end of that side of the lagoon, but I forget what number it is. Overall, if I have to choose between CSSP and Blue Lagoon....I'll make the drive to Blue Lagoon. If I'm looking to make a day trip and be home for dinner, I'll drive the 1.5 hours to CSSP.