It's more a boat than a ferry (i.e. it's quite small to be honest). There is an alternative route round on the train Exmouth to Starcross and vice versa which takes around 45 minutes (but this is more than double the amount of time the ferry crossing takes). The ferry only runs until end of October anyway so this is the alternative route that everyone has to use during the winter months. I'm not defending them, I really don't agree with their policy and it may not be legal but this is what I'm sure they'd argue.It is unlikely to be legal to refuse someone from a ferry crossing for this reason, as there is a duty to make reasonable adjustments. I don’t know what the size of the ferry is, but it would seem reasonable to me that a person with a bona fide exemption from wearing a face covering could be accommodated but if necessary asked to remain in a separate section of the ferry.
Edit: interestingly I've just found their Facebook and they're saying everyone from age 5+ must wear a mask or no entry. I thought the age threshold was 11?