"Resort casual" cruise wear is the primary suggested attire aboard most major cruise lines, as cruise ships today for the most part are very informal environments. Resort casual wear is perfectly acceptable during the day, all day, everyday, as well as during informal evenings. Resort casual day wear can include anything from your bathing suit and a t-shirt on the way to the pool, to a pair or Bermuda shorts and a polo shirt while enjoying a nice afternoon cocktail on deck. If you are participating in any shore excursions while in port, you will need to dress appropriately for your particular excursion.
For example, if you are going to be hiking through a Hawaiian rainforest, be sure to pack some comfortable hiking books/walking shoes. All you have to do is simply dress according to what your plans are for the day, and most importantly, whatever will make you feel most comfortable and relaxed.
Evenings aboard your ship that are designated as resort casual or informal nights, generally call for just nice, everyday casual attire. Most guests will wear a nice pair of khaki pants or skirt with a polo or button-up collared shirt. And although jeans and t-shirts are fine for daytime resort casual wear, most passengers avoid them for evening informal attire-though they can still be more than acceptable if worn in an appropriate manner.