I know it takes on the order of a whole day or longer and that its price is approximately equivalent to $150. I am also aware that there might be flights to the main islands of Japan for less.
In my searching I found a mention that the ferry stops at Motobu in the north of Okinawa on the way. Naha is in the south. But I can't seem to find any information on whether it's possible to for a passenger to board the ferry in Motobu and, if so, whether the price might be a little cheaper.
(I am also interested in other ways of getting from Okinawa to the main islands of Japan by sea too but have not come across anything else.)