Well this is pretty lame. Prof. Elhauge (HLS) argues that because early Congresses in which framers of the Constitution were heavily represented passed laws mandating that ships in foreign commerce provide medical care to sailors, and that citizens have firearms, it follows that the insurance mandate in Obamacare is constitutional. I think pretty good arguments can be made that the individual mandate is constitutional given commerce clause jurisprudence as it has, uh, evolved. Under an original understanding thereof, it is a much harder lift. But look, Congress has the express power, it hardly could be more express, in the Militia Clauses "[t]o provide for . . . arming . . . the Militia". That's why Congress can require individuals to buy guns -- they are requiring the militia to provide for at least some of its own arms. The history here is also just a google away -- it was a big problem that men would show up with inadequate or no rifles. Militias even back then tended to be pretty weak sisters to regular armies, but they were much beloved by republican form of government sorts. Requiring men to buy their own guns was good for militias in the same way families owning minivans and SUVs is good for carpools. It has exactly nothing to with a mandate to buy medical insurance coming supposedly under the interstate commerce clause.
The professor's argument based on the law requiring medical insurance by companies sending sailors on ships onto the seas may not be quite the same insult to one's intelligence but it is still completely unconvincing. A mandate running to shipping companies that are per force engaged in foreign and interstate commerce is hardly the same as requiring individuals to buy insurance just because at some point they are likely to need medical care. Ships moving goods on the sea is about as paradigmatic an instance of interstate and/or foreign commerce as you can get. A law that says sailors shall have (via insurance) adequate medical care is like saying trucks moving goods across state lines shall have sound tires. How more more in the stream of interstate or foreign commerce can you get? This as opposed to Bob, sitting on his couch playing Angry Birds for the 10,000th time, who unbeknownst to him, is floating in the stream of the interstate market in medical insurance. Here, sailor; there, coach potato. This is the very picture of a misanalogy. Broccoli has nothing on it.