For St. Maarten, I'd reccommend Orient Beach. If you go with an excursion, they'll have chairs, facilities and food for you. We tried to find a better beach last visit and could not. For St. Thomas, I'd do the Trunk Bay Beach excursion to St. John. It's extreemly relaxing and has the most gorgeous scenery. I have never been to Magens Bay, I heard it was too crowded, and we don't like that. You can check the number of cruise ships that will be docked that day on
www.VInow.com. If there are more than 3 ships, I'd avoid Magens. Others have said Magens is nice though and since I've never been there, I couldn't say.