We were in Alaska end of May. Your footwear needs to be comfortable and sturdy. We did a variety of activities and we took a variety of shoes but ended up wearing our tennis shoes almost the entire time. These were ones that we had worn before, I would not suggest trying to break in new shoes on a trip to Alaska. You don't want to have to worry about sore, tired feet. No reason, though, that you could not wear flip-flops or sandals on the ship. Have a great time.