Mark has put it rather succinctly - the French are quite relaxed indeed. If you leave the ball on a mooring
at Marina Royale you will meet the port captain
and he's also the same person who does entrance and exit clearances and is very affable. Friends of mine had a cat on a ball there for a couple of months as well. I cleared in there a couple of days ago and he recognized me and waved away all the paperwork (I can't recall
if they ever actually really checked the previous exit papers...) and we talked about the influx for the Heineken Regatta
On another note, I did have friends flying in (from Europe) and gave them a letter stating that they would be on a boat, but that was just backup - all they really ask is whether one is staying on the French or the Dutch side at airport immigration.
So, to go back to the original question - you would clear your boat in, arrange the mooring fees
and perhaps guardiennage if wanted and that would be it until you return! The moorings, at least the outside ones, are close to Cadisco and it would be wise to ensure that everything not shackled or welded to the boat is locked securely.