Public ferry from Marigot. Taxi to Shoal Bay East - tell the driver you want to go to Elodias, and you want picked up at 4pm.
Elodias rents chairs, umbrellas, towels, snorkeling gear, and they also have showers.
There are plenty of chair rentals on Shoal Bay east - but Elodias is the best, quietest area and big stretch of beach, good food and drinks.
Take cash for the ferry and departure tax and don't forget your passport.
Have a great trip.
Ellen