Of course, the crucial assumption here is that it is possible for humans to acquire a set of reasoning skills so thoroughly and reliably that they will actually apply them to all issues, no matter what. I don't think this is possible, and with this in mind, I still don't think the "waterline" concept is useful.
Keep in mind that this concept was introduced in the context of teaching others. The practical advice is to teach skills that will enable people to give up their false beliefs rather than directly arguing against the false beliefs, both because emotional attachment makes a direct attack more difficult, and because the particular false beliefs you observe are indicators of a larger problem. This does not require the most extreme case that the person will universally apply the skill in all situations no matter what, though the more reliably the person uses the skill, the better it works. If using a set of skill 90% of the time makes a upperbound of 10% probability of making any instance from a class of mistakes, that is not as good as using the skill all the time and never making that kind of mistake, but it is still useful.
If anything, it's dangerous because people may fall into the trap of thinking that they are above a certain waterline, whereas in reality, there are issues where due to all kinds of biases, even the very basic skills are failing them.
Again, this is a technique for teaching. Don't use it as an excuse to trust yourself.
This post grew out of a very long discussion with the New York Less Wrong meetup group. The question was, should a group dedicated to rationality be explicitly atheist? Or should it make an effort to be respectful to theists in order to make them feel welcome and spread rationality farther? We argued for a long time. The pro-atheism camp said that, given that religion is so overwhelmingly wrong on the merits, we shouldn't allow it any special pleading -- it's just as wrong as any other wrong belief, and we'd lose our value as a rationalist group if we began to put status above truth. The anti-atheism group said that, while that may be true, it's going to doom us to be a group exclusively for eccentric nerds, and we need to develop broad appeal, even if that's hard and requires us to leave our comfort zone.
Things got abstract very fast; my take was that we need to get back to practicalities. Different attitudes to religion have different effects on different types of people; we need to optimize for desired effects and accept what tradeoffs we must. We can't appeal equally to everyone. So I came up with a sort of typology.
The Four New Members
Annie