Havana and Varadero or Cayo Blanco
 

If you had only 6 nights in Cuba, assuming you would spent 3 in Havana, would you choose Varadero or Cayo Blanco for the other three, looking for sun, sand, snorkelling and relaxing?

Cayo Blanco is a small island a few kilometers off the coast of Varadero. There are NO hotels or places to stay on the island. It's used for day-trips via catamaran from Varadero. So pick a Varadero resort based on your requirements and check with your hotel tour desk for a day excursion to Cayo Blanco. Other than that, Varadero beach is nice but it's not a great snorkelling beach. Long and flat with no reef. Great swimmers beach though.
If you're in Havana, why not consider Jibacoa as a resort to unwind. It will fit ALL your requirements and is located about 1/2 way between Havana and Varadero. Nice thing is there is great beach, swimming and a reef directly off-shore so the snorkeling is great. Usually, you will find Jibacoa under the Varadero section of travel brochures.
Now, I'm not a beach peson, but I've spend months of time in Havana which is one of my favourite destinations. I love the culture and music scene and just wandering about it's varied neighbourhoods.

Enjoy Cuba.
Steve