I will be in St. Malo with no car. Are there one day tours available to Mont St. Michel or good public transportation?
It looks like there is bus service between the two. Check out this website:
http://www.st-malo.info/mont-st-michel.html.
I'm sure there are also organized day trips.
EDIT There is also the option of taking the train from St Malo to Dol de Bretagne and bus from there. It's not particularly difficult, you just need to make sure the schedules mesh to make the trip work. You can do some research on the tourism website for St Malo and also for Brittany. They can be very helpful.
Sometimes public transportation between locations like this, ends up being more effort than it's worth. So you'll want to look for day tours as well.