But here is my question: If the purpose of all this was to encourage people to do tshuva, would it not have been more sensible for Noah to build yeshivas for baalei tshuva (like Ohr Someyach or Aish Hatorah), or go on speaking tours, or go to the Western Wall and invite people there for a Shabbos meal, and all such activities that those of us involved in "kiruv" typically engage in? Why build a boat in the middle of a field?
I would like to suggest an answer: The best way to persuade others to change their lifestyle for the better is by our own deeds, by our example. Think how we try to inspire our children with the great figures of our history: the Vilna Gaon, or the Chafetz Chaim: it is usually not so much by quoting their halachic decisions, as by recounting their deeds!
There is a Gemara (Yuma 86a) in which Abbaye describes how a Torah observer should behave. If he studies Torah and respects it, is honest in his business dealings, and speaks pleasantly to those around him, people will say: "Happy is the man who studies Torah! Happy are his father and his Rebbe for teaching him Torah!" People will immediately give the credit for his good behavior to his Torah education.
We all know that if an observant Jew behaves well in public, this goes to the credit of his Torah observance; and if, on the contrary, an (ostensibly) observant Jew behaves badly, this is immediately taken as a confirmation of people's prejudices against "frumkeit". The first person has committed a Kiddush Hashem, and the second has committed a Chilul Hashem.
The Midrash tells a story. R' Shimon ben Shetach bought a fish one day, just before Shabbos, and on opening it, found a pearl inside. He rushed back to the store (although Shabbos was approaching) and returned the pearl. The owner protested: "But I sold you the fish, with whatever was inside!" R' Shimon replied: "No, you sold me just the fish, not the pearl!" Whereupon the owner exclaimed: "Blessed is the G-d of R' Shimon ben Shetach!"
If we want to influence the behavior of our friends and colleagues towards greater observance, preaching is all very well, but the best way is by teaching with our actions.