I saw once an advert from Czech, where they offered big cruiser 5nights trip to Korea and Japan, and back. cost was around 6000rmb per person with food. but not all in. 

But single trip , just directly, I have no idea, 

there was cruise ship going to japan back in 2009, i am not sure if they are still running. you may consult a travel agent to find out.  the cost won't be cheap by the way.  more than 6000 for a 6 day trip.

a ferry leaves from the Shanghai International cruise terminal 3x a week.

http://www.shinganjin.com/index_e.php

I would suggest avoiding getting a boat to Japan in case you are then targeted and sunk by a Japanese torpedo, or a North Korean missile that has gone astray in a test.

You have a SOH, right?

Yep, you can get the ferry

http://www.shanghai-ferry.co.jp/english/

I was thinking of doing this before. Apparently you can also go from Tianjin.

I took a ferry from Shanghai to Osaka. It leaves every Tuesday morning and takes about 2 days to arrive to Osaka. It costs me about 1000 rmb with student ID. From Osaka you can take a cheap bus to Tokyo.

I took a ferry from Shanghai to Osaka. It leaves every Tuesday morning and takes about 2 days to arrive to Osaka. It costs me about 1000 rmb with student ID. From Osaka you can take a cheap bus to Tokyo.