Not quite as casual as shorts, but men in khakis and tropical shirts, women in tropical dresses... or what I called Mu mu's... but it was a lot of fun bargaining in Marigot.
I saw the tour as VERY relaxing. As a person who had never been to the islands before, i really enjoyed having someone else deal with the hassle and headache of getting around. Now, for my next cruise... I'll probably venture out on my own, but for a first trip I think it's a good choice, especially if you have smaller children.