One of the main villages in the area called La Venise verte, "green Venice," goes by the name of Maillezais [my-yuh-ZAY]. It's an old place. Over the course of the 11th century, starting in around the year 1000, a great abbeywas built here and became a cathedral in the 1300s. It's now in ruins, a victim of the 16th century Wars of Religion between Protestants and Catholics; abandonment by the Pope in the 17th century in favor of a new cathedral in La Rochelle; and sale to a man who exploited it as a stone quarry from the time of the 1789 French Revolution and well into the 19th century.
La Venise verte is more officially called Le Marais poitevin — the Poitou Marshlands (wetlands, swamp). It's just outside the city of Niort, and not far east of the big town of Fontenay-le-Comte, in the Vendée département of western France. The marsh was drained over the course of the 11th century, and rivers were channeled. The canals nowadays are a tourist attraction. Maillezais is one of the major "ports" in La Venise verte.
You can rent barques (flat-bottom boats) and paddle or pole them around on the canals through the marshes. Or you can ride in a boat with a tour guide who serves as the batelier (boatman) and learn about the history, geography, and wildlife of the area.