I don't know the specifics for LFMN (Nice Côte d'Azur), but KBOS in Boston, Massachusetts, a major international airport which is even more surrounded by water than LFMN is and where the runways go right up to the shoreline, uses a set of buoys to demarcate a restricted security zone in the surrounding waters.
This zone is patrolled by state, local, and harbor police and the Coast Guard, and anyone who enters without authorization is subject to prosecution:
The Massachusetts Port Authority (Massport) began installing 29 buoys around the 6.6 mile perimeter of Logan International Airport today. The buoys clearly mark the 250-foot inner airport security zone and will serve as reminder [sic] to boaters to steer clear of the airfield. The security zone was established by the legislature at the request of the Authority in 2002 as part of the Chapter 90 General Laws Section 61 and is one of many security enhancements Massport has implemented at Logan.
The buoys extend approximately five feet above the water so they are clearly visible from a distance. The buoys are similar to the buoys that mark the main channel of Boston Harbor. Three types of marine buoys of different size will be installed along the 250-foot security zone demarcation line ---- Class III, Class IV and Class IV.
The security perimeter is patrolled by State Police, the Coast Guard, Boston Police, the Environmental Police and the Boston and Winthrop Harbormasters. Only authorized law enforcement and military vessels will be allowed in the inner security area. Boaters who enter the security area for non-emergency purposes will be subject to prosecution and State Police will keep a database of offenders. ["Massport Installs Buoys Around Logan Security Zone; Multi-Agency Enforcement Underway" (Massport press release); my emphasis.]