Since you are chartering a catamaran you’ll need even less clothes. During the day you’ll usually be wearing swimsuits and t-shirts. You’ll need no more than 2 bathing suits per person. Same if you stay onboard in the evening. If you go ashore for dinner you’ll most likely be in very casual beach bar attire unless you are dining at a very high end resort in which case you switch the bathing suits for a golf shirt and a nice pair of shorts or khakis for the guys and a simple sundress for the ladies. Don’t over pack.