In Casino Royale, the town is a fishing village North of Dieppe, it is near the mouth of the Somme and has a beach. Le Tréport is the best fit as any further south there is no beach and further north there is no fishing port. Le Tréport was also in the Seine-Inférieure département (now called Seine-Maritime).
It is harder to pin the location in OHMSS down - the book clearly points to the location being Le Touquet but Le Touquet is separately mentioned in the book. There is no nearby coastal town north of Le Touquet so I have chosen Stella Plage. However, if Bond and Tracey were both driving to a location south of Le Touquet, why did they drive through Montreuil?
Good supporting info here and as always you can see all the Bond locations on the Google Map.
