I visited the Phare de Cordouan in July 2021, just one day before the site was inscribed on the World Heritage List. The ICOMOS evaluation was clearly positive, so I had no doubt that it would be the first new WHS on my trip through the South West of France.
Different from Els, I opted for a boat tour from Royan. I pre-booked a spot on the boat of La Sirène Cruises. Clyde reported some inconveniences with La Sirène in his review below, but things have obviously changed over the years. In 2021, booking online was easy and convenient with secure payment by credit card, no phone call required, you only have to give your phone number in case the tour has to be cancelled.

No matter which operator you choose, the tours are more or less the same: first with a big boat, then, split into two groups, with an amphibian vessel to the tower, and on foot across the sandbank back to the boat, wet feet guaranteed.
I would recommend being in the first group, especially if you want to take photos unspoiled by other visitors. I was one of the first at the lighthouse and with only about ten visitors, one of the keepers started the first tour. The interior was a surprise. I had not expected such magnificent decorations, sculptures and marble floors. There is even a royal apartment, but no monarch has ever stayed there. In the basement you can see drawings showing how the building has been remodelled and extended over the centuries.
After the tour, you have plenty of time to view the lighthouse from the outside and take photos from all angles. It is an impressive construction, a beautiful blend of Renaissance and Neo-Classicism. And it is a bit strange to find such a pompous building on a rock kilometres away from the coast. You would expect a more functional construction. However, it was definitely intended to express the wealth and power of the royal dynasty.

Usually there is only one tour per day, sometimes two, the departure times depend on the tides. My tour started at 8:15am. Tours are only in the warm season, from Easter to October. Several operators in Royan offer boat tours, but be aware that not all of them stop at the lighthouse. So make sure that access to the lighthouse is included.

The unique location and the trip to it add a lot to the experience of the visit. All in all, it was a highly enjoyable visit and the Cordouan Lighthouse is a worthy addition to the World Heritage List.