Leave the dune from the beach, not the car park. 20 minutes back to the centre of Arcachon by sea.
Leaving the dune by road means climbing back to the car park, queueing to exit, then the D218 through Pyla-sur-Mer — at the worst hours of the afternoon.
Walk down to the beach, board, and be at the jetée Thiers 20 minutes later. The basin stays sheltered the whole way.