Hmmmm, Well, I would certainly pay with a credit card. That way if good old United Airlines bites the dust, you can dispute the charges. I would suspect that NCL will do the documents, but I am not sure on that. Also, I would check to make sure you are comparing apples to apples. Good old United Airlines may not have included taxes, port charges, service fees, who knows?
With tounge in cheek, maybe United has blocked space for a "United Airlines Cruise". You know, meals are not included, but you can buy one one the ship. There won't actually be restrooms in the cabin, but you can use the ones aft on the ship....but no lines can form at the forward restroom. I am sorry, you can't use your cell phone either.
You have to admit that it is odd that an airline in bankruptcy is offering a better price on a cruise than the cruise line itself is. NCL has come out with a "no discounting / rebating" policy, so I wonder about it.
Let us know how it progresses.