Hm well thats difficult
you could try this site
Transportation - Sea - International Ferries and call the phone number there (or write a mail) and hope that someone speaks english. Otherwise i would go to a Tourist Information or get somebody who speaks Japanese to book it for me.
Theres also this page
Official Site of Korea Tourism Org.: Ferry Terminals wich has some Phonenumbers, but its from Korea, so you can maybe only book Korea -> Japan and not the other way.....
Well, good luck anyways, thats the best i could do without beeing able to read Japanese
No seriously, ive just stumbled over the page (the first Link in the first post) and got all Information from there.
Firebird