Only just noticed this one.
The guidelines specifically state that time spent on a ferry can be counted as a Period of Availability, although, logically, most people would count it as a Break. I presume that they have allowed it as a POA in respect of short trips such as the Torbay Ferry, or the Fowey Ferry. An artic on the Fowey Ferry 
Now that would be interesting.
If the time spent on the Ferry is not less than two hours each way, then you could count it as a Rest Period and do a 16 hour day, every day.