@Night Ripper,
You aren't some language revolutionary here bucking prescriptivism ("you" isn't, and isn't going to become, "they"), you simply had your narrative point of view mixed up and it's just a simple error really that you could have just as easily copped to or ignored the correction about.
Trying to save face by invoking the mantra of descriptive linguists (which is right about descriptive linguistics without saying anything at all about your error) isn't going to work for anyone but you and you needed no convincing anyway. It just makes you look desperate to save face about anything when you previously just had a glorified typo.
You were much better off than the typo.
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As to your question, I think a big problem people have with ethics is in seeing things as black or white, right or wrong as you phrase it. I think a better way of looking at these things is as a spectrum of more or less ideal between right and wrong. Not all wrongs are equal so it can't be a binary choice (unless I"m mistaken about you not being silly enough to stipulate the nature of ethics themselves in the question) and there can be a scenario where there is a better choice whose alternative is not an unethical choice.
So while I think that ignoring self-interest to put others first is more noble, and a greater societal ideal, I do not think it is unethical to fail to do so. However, your scenario is a bit more ethically complex than that, and involves actually taking action to harm the other person with a directness that I consider direct responsibility that can't be blamed on circumstance (e.g. you can't say the water killed him, this is direct enough where you are), and I personally think that this is wrong to do. I think it is much less wrong to do to kill the other man for your own survival than for, perhaps $50 bucks, but it is still self-interested homicide even if the situation is grave and a matter of survival to you. Your motivations are understandable but this only makes this act less wrong, not right, to me.
So in a choice of kill or drown I drown. But in real life (without the false dilemma, which I understand why you use but must point out) I'd likely not choose either option and split the difference, not voluntarily giving up the plank and drowning but also not trying to keep the other person off of it.
If the person were to try to keep me off of the plank, and threaten my life in any way I would have very little compunction about fighting for my life, even if it causes him to drown. For many of the same reasons I would find it unethical to try to drown the other man for my own survival. But without a false dilemma of 'kill or be killed' it's bloody unlikely that I'll just decide to drown and I'm also not arguing that one must place the survival of others above themselves, just that taking direct actions to kill someone is wrong to do, even for your survival, unless that person was the one threatening it.