You have three ways of gowing through the Mont-Saint-Michel bay:
1. By car: It is not an excursion since no cars are allowed in the actual bay even when the tides retire; from the south of the bay you can reach the Mont-Saint-Michel itself for daily visits of the monastery.
2. By foot: A guide is mandatory, don't try this on your own, the bay is known to be home to tricky quicksands; regular departures from many small towns around the bay.
3. With horse: the one I favor the most, a 1h30 (sometimes more) horse back ride. *Promenades Equestres Bernard Grivel - 50Euros
All three ways will take you to the monastery if you want to visit it. Reservations for excursions are often needed as tides have to be taken in consideration... The legends says that many armies trying to invade the bay and not knowledgeable of it, were drowned when the fast, furious and forcefull tides rolled in. Make sure to see this spectacle once you get to the top of the monastery.
Unless you're an expert on tides or an accomplished geologist, do not attemp to cross the bay on your own, either by foot or with a horse. When I stayed at the "Le Moulin" B&B, one of the stories the owner told me about was about two 14 years old adolescents of the area that died in the late 90's, drowned by the tides that came in... they had previously gotten stuck in quicksands of which they couldn't get out. Although they know better and constantly hear those stories, some young people often dare the bay... which shouldn't be done.