Phuket is hot and humid, so pack clothes that befit the tropical weather climate. Several pairs of comfortable shorts in light material such as cotton or soft denim. Bring along several cotton t-shirts, shirts or tank tops that are loose and comfortable. Ladies can pack a few flowery or cotton dresses. These will be perfect for the evenings, during dinner or just a stroll on the beach. You should, of course, pack something extra nice, like an evening dress or shirt if you are on a romantic holiday or if visiting a fancy restaurant.
In terms of swimming wear, bathing suits will be a given, as you will be travelling across the many beaches for sure. However, don't forget to pack a few extras and a nice cover-up, like a beach sarong. An extra will come in handy while the other one is drying. Don't forget to pack a long dress, sarong or loose cotton trousers if you want to visit the temples.