The Baie de Somme is home to the largest colony of harbour seals in France, and watching them haul out on the sandbanks at low tide is one of the best things to do on a visit. The seals concentrate near the Pointe du Hourdel, a few kilometres west of St-Valéry on the open Channel side of the bay; at low water several hundred animals can be seen at once, basking on the exposed sand.
You can walk out toward the seals on the rising tide from the Hourdel, but only as part of a guided trip — the sandbanks shift and the tide comes in fast and silent. Several local outfits run walks from St-Valéry and the Hourdel; check with the tourist office in town. The standard advice applies everywhere: stay well back, do not let dogs run, and bring binoculars rather than trying to get close.