There are 3 ferry routes operating between Fukuoka and Japan offering you combined total of 18 sailings per week. Hankyu Ferry operates 2 routes, Shinmoji to Kobe runs 6 times per week & Shinmoji to Izumiotsu about 6 times weekly. Tokyo Kyusyu Ferry operates 1 route, Shinmoji to Yokosuka which runs 6 times weekly.
As the frequency and duration of crossings on some routes varies we would advise that you do a live search for crossings from Fukuoka to Japan to get the most up to date information.