Hitchhiking across the Yellow Sea

I'm in Shanghai and i need to go to Incheon/Seoul - or anywhere in Korea, actually. I'm on a super-tight budget and I don't want to spend the cash on a ferry; there's no ferries from here to Korea anyway, and i don't want to go north in China.

So I had the idea to get a ride on a cargo ship - they have plenty of boats, but I went to the office of one company and they didn't say yes. They told me that Chinese customs wouldn't let me leave the country this way - is that actually true? (I suspect it was a face-saving thing, blame it on customs instead of having to tell me 'no', because surely they have a passport control at the port).

Has anyone done this before - hitchhiking on a boat? Who should I contact and how can i increase my chances of getting a ride? Where in Shanghai should I look for the people who could help me with this? I'll put up with not having a cabin or a bed, obviously, and i'm not picky at all - the idea is to have a small adventure and save money, anyway - but i don't know how to make it happen.

Your plan isn't going to be workable, at least not legally. "Hitchikers" equals "stowaways" in today's world, and that's a no-no. Some cargo ships have facilities for a few paying passengers, but this would be more expensive than going by air and definitely by ferry. Cargo ships will no longer take on "hitchikers" due to immigration, Customs, and insurance regulations. No responsible company or captain will do what you propose, it would get them in big trouble. The Chinese Customs line you were told is true but only one obstacle. Port immigration/customs is available but for registered crew of ships only. If you want to go to Korea, face the fact you will have to either fly, or get yourself up to Qingdao or somewhere and get a passenger ferry.

Forgot to add: the only way to "hitchhike" by boat these days is on a private pleasure craft, which is very doable in some parts of the world, but the Yellow Sea in winter is not likely to be one of them. You'd have to hang out at a local yacht/pleasure craft marina and find someone going your way who would take you on--even then, it's courteous to pay some cash for extra provisions, etc.