we took the Swan over a couple of years back during christmas peak. A single recliner seat was around $80 each way, but adding the van added $400 each way.
We were over 2.1 high with the roof box on, but had no problems. You just need to let them know and they will reserve a spot in the higher section for you. (the web site does ask, but I did it verbally).
I'd have no hesitation doing it again. Its costly and probably still works out better hiring, or staying in cabins etc, but then do you get everything in a renatal van that you have in your own, set up *your* way etc etc etc.
I also booked a morning sailing, so I got in in the evening, but had the wife and kids take the 50 minute and $39 flight over later in the day. I got off the boat and drove down to Launceston and met them there. I had all the luggage, so they didn't need to take anything with them. We did the same on the way back. The boat's a bit boring by yourself, so I ended up upgrading to a cabin on the way home, had the laptop with me and started doing some work (the joys of self employment)
Andrew