We wore casual comfortable clothing for shore excursions on the east coast. And be sure to wear conmfortable walking shoes. There isn't much opportunity for swimming. A lot depends on what excursions you have planned. We purchased the shore excursions to the Boston Naval Yard, and everywhere else we went on our own. You need to take a lightweight jacket in case of cooler weather. Your black dress sounds fine for a formal evening. As for looking out of place, you will see everything from evening gowns to dressy pantsuits for women, and tuxes to casual pants and dress shirts for the men. Just enjoy yourself and don't worry about looking right to anybody else.