no dressy clothes needed at all. You can pay a modest premium and eat in the classier restaurants, where smart casual is nice, or just eat in the buffets, which are fine. I don't know if they're still keeping up standards as the ship is about to go to Australia forever, but I've been on the OVs several times and enjoyed myself.
You may very well find you can do shore excursions cheaper yourself rather than paying for the official ones - that's how cruise ships make their money - just make sure you're back in time, as in theory they won't wait for you.
Take any pills you need as you may not get them on board or on shore. Apart from that, some layers of clothing (it can get cool outdoors after dark when you're sailing) are useful. You don't need soap and shampoo but do need to take your own facecloth, as I recall.