The information everyone has given about the Coho is correct. I take my bike over every other week and have never had a problem. If you put the back wheel in the rack it doesn't move around nearly as much.
The only comment I wanted to make is that there is another passenger ferry in Port Angeles that takes bikes. It is called the Victoria Express and I had a very bad experience with them. They load your bike for you. When I took the Express from Victoria to Port Angeles my bike was less than a week old. They made me sign a bike waiver stating that they were not responisble for any damages. Well the first thing I noticed when I got my bike back was that both of my water bottles were completely flat. A little odd, but at first I didn't notice anything else. Then as I looked more closely I notice that there were little flecks on my handlebar tape, I thought it was mud, but at closer inspection it was actually paint. Next, I noticed my bar end plugs were missing. Then the worst part was a big chip in the paint. Did I mention my brand new bike was less than a week old? By this time I was already outside the terminal and I had very little time to catch my bus. I knew I had signed the liability waiver, but I figured I would call later and they would at least give me a free ride since I go back and forth so frequently. I called many times, they just kept telling me to call back later and ask for someone else. Eventually, I just gave up. Even though the Express has a better schedule for me, I still take the Coho and tell anyone with a bike not to go on the Express. Honestly, I probably would still take it if I was going without my bike because it is quicker and runs more often, but I would never trust them with my bike again!