Of course this is a regional thing. All of the edge cases of English pronunciation will have regional variation; that’s what dialects are for.
First off, I’m from southern Illinois, but I spent much of my childhood in southeast Missouri. I suppose I have a vaguely Midwestern accent: I say ‘soda’ and not ‘pop’, I ‘wash’ instead of ‘warsh’, and my trees have ‘roots’ instead of ‘ruhts’. (Those examples of things I don’t say come from my grandmother, who has spent her whole life in western Montana.)
Anyway, I pronounce what and watt differently enough I can tell them apart without feeling for aspiration: They sound different to my ears, mainly because I use a different vowel sound. ‘What’ has an /a/ pronounced near the front of my mouth, whereas ‘watt’ has an /a/ pronounced higher up.
‘Which’ and ‘witch’ I can feel to be different, but I use the same vowel for each so I can’t hear the difference. Same with ‘why’/‘wye’ and all of the others.
I pronounce merry and marry differently, albeit not very, but marry and Mary are homophones in my dialect/accent. Pin and pen aren’t, though; not even close. Cot and caught are homophones, as are hock and hawk and horse and hoarse.
I’ve read that it’s not a part of any American dialect anymore, though you could almost call “elementary schoolteacher English” a dialect, given how drastically different it is from the way the rest of us speak. I have only ever heard it as a learned sound among people making an effort to be excruciatingly correct. I think it’s an affectation.
I wish I knew of a place that had good maps of American English dialects - where’s the isogloss between “mary = merry = marry” and “mary != merry != marry” dialects?
Data point: Chicago. I was definitely taught in grammar school that there was a difference, although I do not know of any Chicagoan who makes the distinction. I was taught that “wh” words like “What,” “why,” “where,” should be pronounced, roughly, “hwat,” “hwy,” and “hwere.” I’ve always associated this sound with Southern dialects.
I was taught this in Peterborough, Ontario, as well. But I don’t know anyone who actually pronounces it that way. I associate it with British people of my grandmother’s generation (birth 1890 to 1910 or so).
Different. Stock has a short o. Stalk has an aw sound.
The same. I will say them differently to myself when making sure I type out the correct word, but they are pronounced the same, and Marriam-Webster agrees.