I spent time in both Guardalavaca & Santiago de Cuba in March with a group from the UK, and would offer the following comments on Cuba and its diving.
We enjoyed the country but the basic infrastructure is poor - bad roads, nothing to buy, derelict housing - but the people a generally great, although they are always up for a 1 dollar bill!!!
I must admit to finding the diving disappointing - the corals were great and good condition, but most reefs topped off at 15-18 meters, so you couldn't just potter around gassing off on the top of the reef like you would say in the Red Sea.
The fish life was also extremely disappointing - almost non-existent, and obviously fished out. Like I said we were in 2 different centres on different sides of the island, so I cannot believe that the rest of the island will be any different. Pals of mine have been to both Varedero & Guardalavaca and they say that the former is even worse.
I would say that the dive centres were very friendly but you do have to take responsibility for your own safety - their kit is predominantly government owned and not always to the greatest state of repair, and the diving practices can be variable.
Don't get me wrong we had a fantastic time, but given that the Red Sea is only 5 hours away from the UK and is in every way marvellous, I don't think I will be returning to Cuba.