I have not been on a cruise before, but I have been to Barbados twice, stayed with family and honeymooned there about 15 years ago. I don't know if there is a shore excursion to the malbu beach, but it was one of my fav. We got to go on a tour of the rum factory and then spent the day on their beach.
There is also acra beach, it is really nice, but I don't know how much a cab there would be, we took public transit while there.
Interesting fact: There is not priviate beaches in barbados. You can walk the shoreline and not be on priviate property.