I always hate these scenarios because of the ridiculous elements involved. Can we boil these down to their essence. You have found yourself in a situation where you value stealing someone’s property more than their right to not have their property stolen. You can always steal someone’s stuff, whether this be on a whim or out of necessity does not change the fact of theft. Most people would help a man in such dire need, they do not require any form of law to do so. Conversely, they should not be legally obligated to provide assistance.
Let us say that the man in the desert is a horrible asshole and would gladly watch you die rather than part with his bottle of water. You raise your weapon and take the water. You are now a thief and a robber but what is owed? A bottle of water and perhaps some other remuneration to compensate for the trauma you imposed on the vendor. Depending on the severity of your situation this will be a small price to pay for you. Ethically speaking there were no good men in this situation but all parties can be made whole after the fact.
The same goes for the empty house stocked with food. You may do a wrong in the short term but you can still pay for damages and replace what you took after the fact.
We should not be under the illusion that everyone everywhere will behave ethically nor does this represent any kind of contradiction with Libertarian philosophy. We are quite aware that men are not angels.
I have another thought on this. Is it reasonable to assume a vendor in the desert is selling 1 million dollar bottles of water unless there are people in the desert willing to buy million dollar bottles of water? Not a great business man here. Is it not more likely that if you encounter a man who is not willing to part with his water for anything less an absurd amount that perhaps he to is actually in dire need of that water.