Regarding the weather in Japan in October: In Tokyo and other "northern cities", it was pleasant. I did wear jeans, knit tops with a hoodie or lightweight jacket. Our particular cruise went south to Okinawa, Jeju Island, South Korea, Kyoto, and others. It was warm enough for shorts during the day while on a tour. You may have to wait until closer to your cruise and check the weather forecast. We lucked out, but your cruise could be cooler.