If you are leaving from the US and returning to the US without visiting any ports, I wouldn't think so at this point. This is a very good question about the requirements.
Hopefully one of the travel professionals or someone will know for sure. It would certainly put a damper on the casino boat business here.
Another reason for proof of citizenship - although the cruise is "to no where" there is always the possibility something could happen (medical or equipment problem, etc.) which required the ship to stop at a foreign port.
Passports are strongly urged for our upcoming cruise to nowhere in May. It is uncertain when the land and sea portion of the passport rules will take effect.