Pinel Island is a short ferry ride from Cul de Sac. There were plenty of chairs when we arrived in the morning. By noon, the selection was much more limited to plan to arrive early to get your pick. The rental for 2 chairs and umbrella at Yellow Beach was $20, which seems to be the going rate. The water was very calm there. There are even a few tables in the water, a great place to enjoy a drink.
The chairs at Yellow Beach do not lie flat, so you might want to go over to Karabuni if you plan on being there a while or want to lay on your stomach.
Lunch at Karabuni (hamburger, ribs, and fries) were very good. It does get crowded so a reservation is recommended.
Be sure to visit the store on the island. They have a lot of nice things at reasonable prices.