I recently took the Blackball ferry from Port Angeles to Victoria and back last month, and the return trip the ferry was reallly rocking and rolling. It was bad enough that I was very concerned about my bike, since I hadn't used the blocks on this trip. The cheesy little rope that you use to tie the bike with really wouldn't keep the bike from going over once it started.
While moving, all passengers have to be off the car deck, so I couldn't check the bike until we arrived in Port Angeles.
Once I was able to get on the car deck, I was relieved to see that everything was fine. I'm now convinced that it would take some extreme rolling of the boat before the bike would keel (ha ha!) over.