If I am reading multiple place on the Interwebs you may be out of luck. There were ferries running regular scheduled service from Taiwan to Okinawa but they apparently went bankrupt(OpenJourney confirms this as well).
If one to believe Wikitravel there is an irregular service offered by Star Cruises but I can't confirm through their website this to be true.
This site still refers to a ferry service on a now bankrupt line but also gives a referral to another line which may provide ferry service to Okinawa from Kagoshima but that's about it.