Having just finished MARS II (or NETP-O as they now insist), I can say with authority that:
1. It is a 'booze cruise' - for the most part. There were some nights I was up until 0200 studying for a test the following morning, but only because they had only taught us the material that day. Doing MARS II right after BOTC is heaven... No curfews! No weekends randomly taken away! And you can finally keep food in your room! (a very exciting treat, trust me.) There is plenty of work to do, don't get me wrong, and lots of stuff to learn, but in comparison to BOTC - easy.
2. It's not always - nor mostly - done on the Oriole... Reg Force guys may or may not get a change to sail on that ship, but only if you're in that select class. I'm a Reservist, but we had a couple Reg Force guys on our course, and we only got to sail on YAGs. There was, however, a section on the Oriole in our Ship-knowledge books that we had to fill out on our sea phase, so it is entirely likely that you may get to sail on it. But only if you're Reg Force.
3. As to what it involves... Learning basic naval stuff, such as terminolgy, small boats, common evolutions (such as towing, line transfers, anchoring, etc), ships' organization. Also NBCD, firefighting and flood control (fun!), a sea phase (supposed to be 2 weeks but they cut it down to 9 days for us this summer) and a trip to the range. Sword drill, as someone mentioned... You'll be in your salt and peppers for 6 weeks, so be prepared to do a lot of ironing or have 5 short sleeve shirts. It's not tough.
4. You're entitled to wear your cap badge after you've graduated from BOTC, or so says the CO of Venture. Even if you're not technically finished BOTC... (Reservists now have BOTC by correspondance during the fall... heh... no more comments.)
And yes, RNW, the failure rate is really quite high. Only 2 of 7 passed MARS IV this summer at Venture, and 25% have already failed the MARS III course now running (also at Venture), and it's only half done. It's brutal because you have to be good at what you're doing. Running ships aground or colliding with civvie vessels is really not the kind of press the CF needs.