I haven't been on it but online reviews call it small (600 passengers, which is medium-small, I would have thought) and informal, which means you can get away without taking black ties and ballgowns and whatnot. Smart casual for meals should be fine, probably won't even need ties for men if you don't want to. Does the brochure give any hints? Or the travel agent? As you're heading north you'll probably need to wrap up a bit.
Speaking of tips, though... you'll probably have to leave some at the end (they'll tell you how much!) so keep some money aside.