As regular travellers on the French routes, we do as mark and Wendy have suggested. I notice from the Irish ferries website that passengers (with a few exceptions) are expected to wear face masks in congested public areas on the boat. They seem to need to be worn in French territorial waters and the port of cherbourg.
Reports of motorhomers already in France suggest that like home, masks are not essential in many places, but some shops, restaurants etc ask for them to be worn. We head out mid August and intend to keep a mask in our pockets along with the personal sanitiser bottles and observe local usage.
Davy