I'd suggest comfortable, and possibly waterproof or water resistant shoes, such as sneakers. If you plan to do water sports, kayaking, etc., take Tevas or similar. If you plan to do serious hiking, take waterproof hiking shoes. But for general, all-around excursions, a good pair of sneakers should "foot" the bill.
You can wear the sneakers onboard the ship, too, during the day, plus a pair of dress shoes for night.