Cuba is pretty much the wild west when it comes to diving. CMAS is the most prevelant agency - but as has already been pointed out - wave a referral card at someone and chances are you'll end up in the water - it's pretty much the Darwinian approach!
May I suggest for your own piece of mind that you complete your open water course here before you go - you won't be able to do it out there.
Neptuno is a great wreck dive in about 10m - just off the coast of Varadero - make sure you don't miss it and give my love to the Congers! lol