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I found this twelve-year-old post on a Lonely Planet forum:
Weekly ferry services operate between the Taiwanese ports of Keelung and Kaohsiung, and the southern Japanese-island of Okinawa. The ferries arrive in and depart from Naha, the capital of Okinawa, often stopping at the Miyako and Ishigaki Islands en route. Fares for the 20-hour journey range from ¥15,600 to ¥29,300 one-way. Please contact the Arimura Sangyo Company for further information.