I send a message to "Anse Marcel Beach" via Messenger in Facebook. I always make a reservation, even if it's not high season, as they had a reputation in the past for turning people away who didn't have a reservation. If I remember correctly, chairs/umbrella are about 30 Euro. It can be a spendy day but well worth it in my opinion. Tell them approximately what time you want the beach loungers and what time you would like to eat lunch, as well as number in your party.