Hello, I'm wondering about this also.
Reason is that I'm not sure what is the best course of action should there be a ferry cancellation affecting my avios purchased flights. (Ferry should arrive at 11am and flight is at 18:35 the same day)

Flight is with JAL (nobody had any information or experience on the JL forum) but purchased through BA.

If we become aware of possibility of a ferry cancellation on our return and can't secure a place on a ferry leaving the day before, then it seems we can change the flights (outside of 24h deadline). But if the ferry cancellation only becomes apparent on the day of travel the not sure what we should do. JAL website doesn't go into what happens when travel is disrupted using award bookings made with non JAL FFPs.

Reason I'm particularly keen to understand what to do is because the standard ferry (which can sail in up to 4m high waves) is being serviced on our dates. Only the fast ferry (up to 3m high waves) is in operation. The possibility of not being able to return to Naha is therefore a little higher.