There's nothing worse than holding the door open for someone coming behind you in a shop and them breezing past you without saying thanks.

I'm not so sure about that. I tend to hold doors open and so forth without even thinking about it, it was built into me while I was growing up. If someone says thanks, that's nice, but if they don't that's fine too. I'm not doing it to be thanked. And often enough when someone has done the same to me and I've been preoccupied or whatever, I've not said thanks.

When people protest vocally about not being thanked for some elementary act of courtesy, as they sometimes do, it somehow seems to undermine the whole value of the act - it seems as if they are measuring out their courtesy by the inch.
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As for stuff like "water" - "woah", the thing is, however we speak we are going to sound weird and ridiculous to people in other places and in other settings, and it applies both ways. Obviously we owe it to the other people in such situations to accept that however ridiculous we both may be in each others eyes and ears, we should accept the difference and try to put them at their ease rather than ridicule them. (That doesn't mean we should feel it wrong to be inwardly amused.)