Casual all the way. The polo shirt/khaki ensemble is about as dressy as it gets for men -- choose your own outfit accordingly. If you'd like to wear a sundress/skirt that fits in. If you want to wear casual (but nice) pants/shorts outfits that's fine, too. The same applies for most restaurants outside the resort as well. Seems like the whole state of Florida has very few places where dressing up is expected.