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All beaches in the USVI are open to the public by law. Whether or not a non-guest can rent equipment or water toys at Grand Bay is another matter. Don't have the answer to that and you may have to check directly with Margaritaville.
The beach is OK at Grand Bay but there are much nicer options (Coki, Sapphire and Smith Bay/Lindqvist) close by.
As was stated above, the beach is open to the public. You may also use the restaurant, bar and restrooms. It is my understanding that the beach chairs, pool, pool bar and equipment rentals are for resort guests only.