We're making a raft, 8ft x 6ft, and wanna know if it is legal to simply build it and sail it down the Chicago river? Does it need to be registered or anything like that? (it isn't, and no one has a boating license).

Yes, any watercraft must meet U.S. Coast Guard safety requirements, and be equiped with all required safety equipment. Perhaps you should contact the local Marine Police in Chicago, since they are going to be the agency in who is jurisdiction you are planning to use your raft. Good Luck!