Not a Haven expert, but my husband wore nice shorts at dinner in the Haven restaurant on the Joy last year. On his feet, he had on Keene sandals. Never an issue with that restaurant or any others that we ate at. Next week he, along with our son, will also be wearing nice shorts and sandals while dining both in and out of Haven. The only pants they will have on them will be the jeans they wore on the plane flying out from New York to Puerto Rico. I personally think their shorts with a nice shirt (my husband is partial to Tommy Bahama shirts--both collared and non-collared) is a nicer look then some of the jeans and sweats I have seen.
So, you do you and everyone should do what makes them happy. My steak will taste the same no matter what someone is wearing at the table next to me.