Mont Saint Michel bay has the best tidal bore in continental Europe (the Severn estuary in the UK outranks this one I believe). and you will see a bit of it if you visit on August 31, but better on September 1 and 2.
The red marks on the tidal charts indicate when the tide has risen so far that the Mont is cut off from the mainland. For example, on August 31, the period of insularity will be from 8:20 pm to 9:50 pm.
If you are not staying overnight on the Mont, you may want to leave about 2 hours before the 8:20 pm cut off. That way you can view the advance of the incoming water from the mainland. and then head back to Dinan.
More on tidal bores: MSM tidal bore
If you prefer to visit on September 1 or 2, I can send you the link to the MSM city hall website (French) which has the cut off times detailed for locals who live and work on the island.