Take as little as possible! On board, we dress very casually and shorts, T-shirts and swimwear are the norm and you might want a pair of jeans and nicer shirt for evenings in towns. Together with some underwear and a toothbrush… this is probably all you need!
Oh and there is no room to store suitcases on board, so try and put it all into a soft bag that will squash down after its been unpacked.