It is hard to give a precise “average duration” for berth “amodiation” in France. Every port in France, no matter small local port or large commercial port, has its own rules and regulations concerning berth allocation.
Lease agreements on berths have long been negotiated at ports, they can be for decades at berths in ports (the duration of which will be strongly dependent on the port location (Mediterranean versus Atlantic coast) and on other types of agreements).
Amodiations are eligible marketable, transferring outstanding usage detail. Port authority approval is usually required, and fees may be charged, however.
Lease agreements on berths have long been negotiated at ports, they can be for decades at berths in ports (the duration of which will be strongly dependent on the port location (Mediterranean versus Atlantic coast) and on other types of agreements).
The resale value will depend on how many years of the contract remain (each port have a different amodiation duration), where the boat is berthed, and general market conditions.
It’s a transfer of contractual rights, not land ownership, and requires deliberate legal scrutiny.