To reach Cayo Levantado, there are two excursions offered through RCI which takes you directly to the island from the cruise ship. Although there are water taxis ($10pp roundtrip) that you can take from Samana, apparently they are quite strict with cruise passengers...forbidding you to take them due to an agreement with the cruise lines. Apparently they are hard core about it too, so beware if you are trying to save a few bucks, you may be out of luck.
The beaches are overly commercialized but beautiful, though on days when cruise ships are in port they are packed with passengers. I have read online at Fodors.com that you can buy a $125 day pass to the Bahía Príncipe Cayo Levantado, the island's only resort, and take the resort ferry. I am trying to seek out information from the hotel to see if this is still an option.