On the days when the ship is in port, you will typically find the buffet open for lunch and one dining room with open seating.
Is it worth it? That depends on whether you want to spend to time going back and forth to the ship. I personally wouldn't do it if it involved tendering. That can really use up some time and we always make it a point to do everything we want to do ashore in one tender trip in this case. But if the ship is moored and it is nearby, then we usually go back for lunch. Note I said usually. If the port happens to have a place I'd like to try, then that is what we do. For instance, in St. Martin there are many good French restaurants. In Cozumel I like Pancho's Backyard.