IANAL, but I don't think so. See, for example, the famous R v. Dudley and Stephens case concerning cannibalism at sea [0].
The group was shipwrecked, surviving on a turtle and drinking their own urine. The victim, Richard Parker, was apparently in a coma. Despite these mitigating circumstances, the two defendants, Dudley and Stephens, were sentenced to six months each in prison.
That is a very thought-provoking case, but the article does say "the Mignonette was registered in Britain for jurisdiction under s.267 of the Merchant Shipping Act 1854" (and presumably that jurisdiction extended to the lifeboat too).
My assumption is that even if the victim and defendants weren't British citizens (or subjects) the case would still have fallen under British jurisdiction, whereas if the same incident had occurred on a boat registered to a different nation, Britain would have to rely on the courts of that country to interpret their own laws on the matter.
I suppose it's possible that another country could have in its laws a defence of necessity that was applicable to that precise situation, and I don't know how double-jeopardy rules and extradition treaties interact with the nationality or passive personality principles of personal jurisdiction:
Some countries claim jurisdiction over actions carried out by their citizens abroad, so you might find yourself in legal trouble when you returned home. Also, even if no country claims jurisdiction over events that occur there, that doesn't mean there are no consequences for your actions, precisely because it wouldn't be illegal for the locals to kill you in revenge.
I'm not sure if that's a helpful way of looking at things. Something is legal (for a person) if it is not explicitly illegal. So if there is no law against having a bath on Wednesdays in New York, then we say you can "legally" do so. In a territory where there is no government and no laws whatsoever, it is equally legal to have a bath on Wednesdays, so you would be "legally" bathing there too.
You're right, though, that in normal circumstances when we talk about things being "legal" we mean "with respect to a specific (non-null) jurisdiction". I think that's because places without jurisdictions are so rare that we don't usually think of this degenerate case. Perhaps there needs to be some special term like "vacuously legal", or "anarchically legal", to distinguish it from the more common type of legality.
The word legal itself means that it is authorised by law. "Everything which is not forbidden is allowed" is an established principle of law. Without the law, there is no such provision. Therefore, without the law something can not be either legal or illegal.
That's a good point, and maybe it's not well defined what the default legality is under a null (or unwritten/uncodified/implicit) constitution, but my instinct is that illegality requires someone to enforce it, whereas legality doesn't, so legality is a more natural default.
