I spent a week boating in the French and Italian Alps a few years back - i was there in august. We did one day on the Dranse (outside of Geneva), two days on the Isere, and then several days in the Aosta valley in Italy. The Dranse was our warm-up day - we did a class II-III technical run on the lower part of the river. It was a little bony they day we were there - fun, but not much push or challenge. There is a more difficult creek run above the one we did.
The Isere was really fun. The first day we put in at Bourg St. Maurice and did some laps on the slalom course. Big class IV-IV+ above the course, and the course itself is fast, pushy class III. We then boated the run below the slalom course - fast class II-III, completely non-technical, just fun wave trains. And lots of "hydrospeeders" (basically riverboarders) to dodge. The next day we did the next-lower run on the Isere - I don't remember the names of the put-in and take-out, but the run included a super-fun, continuous big water class III-IV section and a beautiful vertical-walled class III gorge. This was probably the best boating on the trip. Plus, a jogging path ran along much of the run, and there was lots of eye candy inline skating alongside the river the whole day. No hydrospeeders on that run. Also, lots of wine and stinky cheese at the takeout - all in all, a high quality boating experience.
We didn't get to boat in the Durance area. From what I heard, it is great.
If you are in this area for long enough, I'd recommend driving across the petit st. bernard pass and spending a day or two on the Dora Balta in Italy. It's not too far by car - maybe an hour and a half or two hours from Bourg St. Maurice. The run that ends just below Villeneuve is fun, technical pool-drop class IV. Be sure to use the right put-in - we put in just below a tough, tight class V gorge. There was a rafting company at the take-out that seemed to be a place where people were finding rides, etc. I also heard lots of good things about the Sesia valley.
The isere and the dora balta are glacier-fed, so june flows won't be a problem. The flows were lower in the morning, and rose as the day warmed up and more ice/snow melted. We did a couple of huge-water runs on some hot afternoons on the Dora Balta.
It wasn't hard to rent kayaks - I believe the person who obtained ours found them in Grenoble. Unfortunately, I can't tell you the name of the shop. However, i don't know about c-boats. There were some people in c-boats at the Isere slalom course, so you can probably find one somewhere (and be sure to prounounce "canoe" with a proper french accent or they won't know what the heck you are talking about - i tried discussing c-boating with a belgian play boater using my limited french and bad accent, and ended up in a very confusing conversation until I got my point across with cherades). Check
www.eauxvives.org for information on French boating - you may be able to translate the site with babelfish or the like.
Have a great time - I'm going to get over there again as soon as I can.
Matt