[Nyororin]

What they will want to see is not necessarily a return ticket, but rather proof that you are leaving the country before your tourist visa expires. You will see "return ticket" a lot, but that isn't an official requirement. It is just the proof for most of the visitors to Japan.
Your ferry ticket to Korea should give you no problems. A lot of these things are also down to the discretion of the immigration officer, so should they question it at all do show them your return ticket and explain your plans. You will have solid proof not only of leaving the country before your visa expires, but proof that you will be leaving the country soon after returning from the Korea trip.