Like I've said before, don't bring many clothes on a LOB, but bring at least 3 swimsuits and 2 cover-ups so that you can be dry and comfortable between dives. I like cover-ups by Dotti. Other items include a few shorts and t-shirts, but don't forget to bring a pair of sweat pants or yoga pants and a light-weight, versatile jacket because it can get cold onboard at night - if you run into some wind and rain. I've also experienced some LOBs where it was cold in the cabin and hard to adjust the temperature, so I was glad for my yoga pants!
You also need one pair of sturdy footwear, like athletic shoes, in my opinion. Don't just bring one pair of flip flops or flimsy sandals. You don't wear shoes much onboard but you can wear the shoes while traveling and they can come in handy if there are any shore excursions.
They usually try to sell you some souvenir t-shirts and other apparel on the boat at the end of the trip, so that will give you a clean change of clothes for the trip home!