Coinciding with the flowering season, the high season for sailing happens in spring and continues until summer. It is a beautiful experience to watch the villages, castles, cathedrals from water. If you mind the crowds and a possibility of hot days, you should aim for the low season falling between late November and the end of December. It might get a bit wet and cold, but it definitely has some charm to it, not mentioning the plentitude of nice weather days also! Remeber to avoid sailing trips in January and February, since most river and canal ships stop operating at his time.