My suggestion is that when you will be to work in the office that you uses a skirt, a blouse and shoe.
When you to leave the office and shorts will be the beach you can use one, a t-shirt and a tennis (or perhaps a slipper)
When to leave supper with its fellow workers uses jeans, a blouse or (a t-shirt) and perhaps boots.
E does not forget to take clothes for diving since you it is for 3 months.