Who you deal with
At sea, the Directorate of Maritime Affairs through the local ship safety centres. Inland, the transport directorate, with the case handled by the navigation unit at département level.
At twelve passengers
At sea, a Division 241 navigation permit, with no structural inspection required below 12 m. Inland, a boat certificate, where the visiting commission may be waived if you are working in a port area. Your navigation category, from 1 to 5, sets how far from shelter you may operate.
At twenty passengers
French rules include a dedicated section for passenger ships under 24 m, and it carries a named exemption for uninsulated aluminium hulls built after December 2017. That is the single most favourable clause of its kind on this list. Inland, it is a Union certificate against ES-TRIN chapter 19.
Who may drive it
At sea, commercial passenger carriage needs a professional certificate rather than a recreational one. The relevant qualification at this size covers commercial vessels under 200 gross tonnes within twenty nautical miles of the coast, but note that it carries a 250 kW propulsion ceiling.
Hiring it out without a crew
This is the lightest route in France. Hiring the boat out without a crew keeps it a recreational craft under the recreational division rather than the commercial ones, and inland a pleasure boat under 20 m needs no Union certificate at all.
Where the engine matters
We found no general French engine restriction. The 250 kW ceiling on the professional skipper's ticket is a crewing constraint rather than an engine one, but in practice it bites the same way. Above twelve passengers on this country's inland waterways, petrol is not an admissible fuel at all, so the twenty-passenger inland configuration would need a different form of propulsion. This does not apply at sea, and it does not apply at twelve passengers.