I really like Secret Harbor. I usually go for 3 to 4 hours. Nice beach, very good snorkeling to the right. There are a lot of iguanas in the area that add to the entertainment. For a full day, I'd probably target Coki Beach beside Coral World
My family will be in St. Thomas in June (Carnival Legend). I did research on many beaches/attractions on the island (at the advice of my very experienced cruise agent!) and you can check out the link I have inserted for a world of information that you are seeking. We will opt for Coki Beach and Coral World, which is across the road from the beach. Check out the site, I am sure it will answer most questions you have.