The cruise is more than you can ever imagine. Really. It's a party of the Blues. 90% are there for the same reason. The other 10% hang by the pool.
So to answer your questions:
Are there formal nights? NO, but if you want to dress up in some sort of formal attire, you can and should. If you just want to dress up, there are "theme nights" for that.
Is there a dress code for dinner? NO dress code anywhere for anything. Well, I've never seen anyone not wearing clothes. I never, ever go barefoot, but shoes are a plus for some.
Are there assigned seating at dinner? Each cruise has been different, but there are no seat assignments or tables. There may be a time that is shown for arrival (early or late) but that's pretty loose, as well.
What time does the music start/end each day? At sea days, music starts around 10 am or so. At port days, it depends on when the boat sails. I usually tell people who ask that the music is about 20 hours each day.
Are there jams? Am, Pro, practice sessions, you name it, you can find it. Some are scheduled, some just happen. There is a venue set aside for the Am jammers where someone sort of runs it, sign ups for the musicians, everyone welcome. The last scheduled show of the night (which is really in the early morning around 1 am) is the Pro jam where one of the bands or musicians is scheduled and the other Pro musicians join in. It's pretty amazing to see and listen to it, warm wind from the moving boat, moon, stars, clouds. The smell of pizza until 4am....
Is it hard/easy to see the bands? Well, no, not really. Anyone is able and encouraged to be right up to, leaning, sitting on the stage. Depending on the venue, you can be 2 feet away from the musician of your choice. And even for the biggest star on stage where the show is crowded, you can easily go right up front to shoot photos, dance, listen, whatever. Seating is everywhere depending on the venue. The big shows scheduled in the small venues (Crow's Nest, for example) may be difficult to see if you arrive late, but you'll always be able to hear.
Is there a host hotel for before or after stay in Florida? There's a host hotel announced, maybe already. In Ft Lauderdale, Hilton or Sheraton, most likely.
Is there a precruise get together? Pre-cruise party is sponsored by the local Blues Society. It will be announced.
We decided not to go on the JAN'12 cruise. We've been on 7 so far and are cutting back to just one/year. Our choice is the OCT West Coast. The lineup for JAN'12 is great. You'll get a cabin, but the cheap ones go first.
Good luck and just ask if you need more info. If you are on Facebook, take a look at some of the cruiser pages w/photos. JAN Cruisers, OCT Cruisers, etc.