Since the recent opening of the long-distance bus market there is a new option to get to the Canal Du Midi, or to travel one way along the Canal Du Midi by bike. This is the long distance tourist bus. Unfortunately the buses that stop along the Canal Du Midi do not have the option to carry bikes, as they do in other cities in Europe.
Flixbus is a German company and they have 3 stops along the Canal Du Midi, namely Toulouse, Carcassonne, and Beziers. These buses do not have bike storage. A one way trip between Toulouse and Beziers costs 7 Euros per person.
Isilines has 3 stops along the Canal Du Midi, namely Toulouse, Narbonne, and Beziers. They work together with Eurolines ( http://www.eurolines.co.uk/en ), who provides connecting services to countries outside of France.
BlaBlaCar is not a bus company, but it can get you from point A to point B pretty easily as well. You go to their website and enter your starting and ending place and your date. They will search for private individuals who are going the same way as you, and offer a place in their car for a small fee. You will thus be riding with a stranger in their personal car. In return you pay a small fee to pay for gas and tolls. If you have minimal luggage and speak a bit of French, this might be a good option for you. Toulouse to Sete costs about 17 euros per person.