I have never done the canals in France
but I lived in France
and sailed out of Le Havre for several years so I thought I would make a couple comments.
First, don't miss Honfleur at the beginning of the trip up the Seine. It is a beautiful little town and you can pull into the central bassin which is literally in the center of town and it is really a great place to visit. Also, I believe that you can also have your mast
taken down in Honfleur or in Le Havre itself if you wanted to do that before getting to Rouen.
As for details about the canals, you might try posting
on the Yachting and Boating
(ybw.com). There are more British people over there who have done the canals who might have a bit more first hand experience.
As for an (international) certificate of competence (ICC), this is a strange issue. Officially (and in the case that you don't
enter the canals), I think you are required to have one but since it must be given by your home country and no one in the US issues them, there is no way to get one-I tried to get one through the RYA but it was impossible since I am not a british citizen or resident. I never had one, chartered boats in France, owned a boat in France, sailed in France and UK and was never asked to show one.
BUT, since you are entering the canals, I believe (though I could be mistaken) that you are required to have a canal permit
(CEVNI), regardless of your nationality. The chance that you would be asked to show it is pretty high since you have to register (and pay fees) for using the canal systems. The French are generally very picky about whether you have the right papers or not and if it is on their list to check it, they will (at least in this case where you would have a hard time getting around the rules by flirting, offering gifts, telling the right sob story, which can all work from time to time with the French.) Hopefully, someone else can give more information on how to get one of these as a US citizen.
This website has a lot of information but is kind of geared toward UK citizens:
Canal Routes to Med
In any case, have a great trip and enjoy your voyage through France, it really is a beautiful country with very nice people and wonderful food