Cruise ships often stop at various destinations and allow the guests to wander on their own before returning to the ship. For this reason, it's good to pack clothing and accessories that are appropriate for both onboard activities and shore excursions. This means dressing lightly in case it gets hot enough onshore for a swim, but also packing formal outfits like cocktail dresses or skirts-and-blouses combinations suitable for dinner with show entertainment.
If it's casual, then you probably don't want to wear your diamond pendant necklace while strolling through a beach in the Cayman Islands because you might lose it. But if it's formal evening dining, where you'll be dressing up, then you'll want to bring your diamond pendant necklace and some diamond studs.
If you're taking a summertime trip to the Caribbean and you know you'll be doing water sports, then pack swimsuit necklaces. These are necklaces with charms in the form of seashells or other nautical items to wear while snorkeling. A waterproof watch is a good idea, too.