It is not forbidden to fly above ocean and seas in France, but lots of area near the sea are forbidden due to the following reasons :
- In the south of France, and in a all long the riviera coast, you have lots of airports such as Marseille, Nice...
- In some bay of the same area (Toulon for example), the Navy operates military boats, submarines, planes...
- Near Marseille, you have a beautiful coast that is called ‘Les calanques’ that is also a natural reservation. In this area, birds, little bays, pine trees forest... that are often on fire. During summertimes and even spring, this area is often closed even to hikers (to dangerous because of fire).
- Military airplanes do also operate a few miles from the coast but above the sea (flights above sound speed are not practiced above the ground to avoid problems)
- Some beaches are dedicated to ‘No clothes bathing’ ... and drones are not welcome for obvious reasons ;o)
Lots of other places are allowed for drone flights above the sea, if you have a few minutes, you can see them zooming out on the map of Geoportail mentioned above.
Even on the riviera coast near St Tropez you can fly drones in some beautiful spots such as on the map below (red : Forbiddn, yellow : not above 50m, Orange : under 30m, else : not above 150 m)
My advice would be to check before your trip, the area where you would like to fly, and find authorized spots. Print the spot on a paper you would have with you, so in case of a question, you can show that you know the rules and what you do is permitted.
Usually, no one bothers. Some people are just curious and will look at you while flying. I usually talk to them, show the Anafi, take a picture of them with me from above and send them the picture via email...