I agree with Boulogne-sur-Mer, nice historic centre. But finding rooms overlooking the sea will not be easy along the Atlantic Coast, there are way less than you might think. Expect the few will be fully booked during summer, so book as early as possible to make a change.
The seaside resorts along the most northern coast of France are not the most fancy, but during summer with fine weather the number of visitors will bring a good atmosphere. Nevertheless to my opinion you can consider the following places:
Still a bit south there is Le Tréport, with hotels along the coast and more north closer to Boulogne-sur-Mer in Le Touquet-Paris-Plage there is a Novotel and Ibis just close to the beach. With Google Maps I can spot Hôtel Neptune in Berck-Plage. Wimereux just north of Boulogne has hotels close to the beach too.
If you can´t find a room overlooking the sea I think too Boulogne is nice to stay. Worth a stop is Le Cap Blanc Nez from where you can oversee the Channel with The White Cliffs of Dover if weather cooperates. Closer to Bruges the litte but lovely main square "Grote Markt" of Veurne along the E40 freeway is worth a stop too.