Let’s see....if the battery starts burning, it will heat up very quickly. Can you charge a burning battery?
Also, several European ferry companies have banned EVs on their ferries due to EV fire danger. I suggest the ferry tow a large flat barge as the EV parking area. If any of them start to burn, they can just cut the tow line and there will be no danger to the ferry.
I recently saw that Maine is going to get an EV ferry for, I think, the Rockland-Vinalhaven run.