A "heart attack" is the common term for a condition where part of the heart isn't receiving blood flow, and the tissue there is starting to be starved of oxygen (an infarction). That will show up on an ECG as elevation of a certain part of the tracing (the "ST" segment, between when the main part of the heart contracts and when it "resets"). In order to be clinically significant, you need to see that elevation in at least two adjacent leads, and each lead is only looking at a narrow "slice" of the heart, so you need several of them to be sure you're seeing the part that's involved (it's not at all uncommon for a heart attack to only show up on 2-3 leads (out of 12 on a typical ECG)).

A watch cannot provide more than one lead (and the lead it does provide is really useless for detecting ST elevation, since it's a view across the "top" of the heart, which is not an area that will be involved in the infarction). It is not physically possible to detect a a heart attack from a watch.

If you had asked me six months ago if a watch could detect atrial fibrillation, my answer would have been "sure, why not?". Just because one thing is possible with the advancement of technology, it doesn't mean all things are possible...