tri shorts are okay, but if you aren't accustomed to it you'll probably experience chaffing riding in wet shorts, even the tri shorts if 9 miles is too far for street clothes, even if you rinse in freshwater.
the poster above is right, change under a towel or behind a bush, you're in France where they generally acknowledge people have bodies. Speedos in the pocket on the way and on the way back, you're all set!