Having only passed through Barbados, etc on the way to Guyana, where I've travelled for work several times, I can't speak for island dress in general. It did look fairly casual to me. But I can speak to moving through the cities and less-touristed areas in the developing world (and lots of the islands are just that, at least outside the tourist spots). My advice in that context is to underdress - at least until you reach your destination. By underdress I don't mean jeans and a tee shirt, which also screams 'tourist', but rather a very simple pair of trousers and a respectable dress shirt. That look says 'I am here to get around and see things, not to flaunt my money'. As globetrotter said, you're likely to stand out enough as it is, and be seen as rich and foreign. And assuming you do leave the confines of the resorts (which is usually a lot more interesting than only sipping mai tais poolside), you'll likely be moving through areas that are far from wealthy. If you are mainly going to the ultra-expensive 'vacation islands' and/or only hanging out in the resorts, please feel free to ignore the above.