Emerald beach seems like an odd choice for St Thomas. I'm sure no one is going to mind your being on the beach, facilities etc. may be another story.
For my money the best public beach on St Thomas is Saphire. Beautiful uncrowdwd beach, all facilities (bathrooms , outdoor shower etc) and renatals available, several food options and a nice bar. Great "walk in" snorkelling and the view across to St John and the BVI's is simply incredible. Taxi stand out in front of the resort for easy transportation.