Cool topic :-)
A standard ECG is 12 “leads” but you only need 10 physical connections for that (6 on the chest and then 2 arms, 1 leg and 1 neutral which is typically the other leg) – long story
You can record lead I (0º or the vector from right arm to left arm across the heart if you imagine the Vitruvian man from Da Vinci) between your two hands. That’s how the Apple Watch and by now also some competitors work – they record the contact signal on your wrist under the watch, and the other contact from a finger on the opposite arm that makes contact with a second electrode on the watch. Doesn’t matter how you hold your arms/hands. I have one and it works great. If you position the watch creatively you can reconstruct quite a bit of the standard 12-lead ECG see for example here
I guess in theory a recording using two tiny electrodes on one arm only can also record and reconstruct an ECG-like electrical activity of the heart – should be a matter of SNR and processing. I’m not aware of any device that does that today but I don’t see why not. It just won’t be a “standard” lead I because that is defined as the vector across the chest from right to left in the horizontal plane.
I think most of the features on a smartwatch (especially the ones related to aviation) are just toys compared to the same features on a phone (which is still a toy).
I keep my phone in sight when flying mostly to look at the navigation.
I tried a smartwatch a while ago and since I am the entire day in front of the laptop and get notifications on the phone as well I find it too much to get the same things on the smartwatch as well.
This video describes how a watch picks up an ECG, and it describes what I posted above i.e. one contact is the base of the watch and the other is taken from the other hand touching the side of the watch.
It doesn’t describe a watch which acquires the signal wholly via its back. From an electronics POV I’d say that is in the realm of fiction – like Tom Clancy describing detection of a human (soldier) at a distance of some tens of metres, from the EM radiation from his heart, generated by the pulsating blood flow in the presence of the earth’s magnetic field. Same principle as EMP but, ahem, somewhat weaker 
All these effects must be real, in terms of “some” electrons moving about, but they will be so far below the noise floor…
More simply, I cannot see how the signal from the heart could be appearing across the little patch of skin under the watch. The only way I can see is if you held your hands together
and then the patch shown would see a voltage gradient across it.
I’ve been skeptical about smartwatches for a long time. Saw them as a phone extension with a crippled user interface. I got my first one when watch-eSIM was introduced in my country. One catch is that watch eSIMs don’t do roaming, even though the base SIM, from which the watch SIM is derived, does. Maybe that is different at other operators.
Aviation. I have not found any aviation uses other than simultaneously showing time in the home, local, and UTC zones. It is helpful for anyone changing time zones. This is my default face, with a graphical display of time remaining in the day.
(local same as home presently).
Kitesurfing. I used to carry my phone in a plastic bag zipped into my life vest. Now I don’t have to. I have the phone and messages accessible at all times on my wrist.
I also don’t take the phone when walking my dog, biking, or swimming. Yet I am still connected and can call for help.
Siri. I use Siri to open the house door, the gate, the garage, and a few more things. Talking to your wrist is more accessible than pulling out the phone or looking for it.
Sleep monitoring. This has been improved lately. Able to discern phases of sleep by monitoring pulse and breath. Also, general health monitoring improves when wearing a watch: counting steps, step length, equilibrium, breath frequency, pulse, O2 level, and others. Trends are created over time, and warnings are presented when they change. ECG is there, but it’s low-res. If you present it to a doctor, they will ask you to come for a real one. There are automated warnings when atrial fibrillation is detected.
(sleep phases, with the lowest being deep sleep)
(ecg)
Swimming/diving – the new models have water temperature and depth displays.
To summarise, they are not really useful for flying, and you can surely live without them. They have some other functions that one appreciates over time.
edit: can be used as a pulsoxymeter, but a real one is more accurate.
For flying the Watch app I use the most is the MetarTaf app I developped :-) It’s been downloaded 9k times, mostly in the US which is a good representation of the GA population, although I’m more doubtful about the 400 downloads from Russia, they’re probably just looking at vulnerabilities !
Because I don’t collect data I don’t know if people keep it up but the API which in the back serves the data is called about 20k times per day, so I’d guess a good few hundred people use the complication to have updates visible at all times.
Justine bought a Fitbit watch and the ECG does work
and exactly as discussed i.e. you have to touch the sides with your other hand to pick up the signal.
Indirect aviation use of a smart watch. Heart rate during a flight.
Helpful for apple watch data export: https://www.ericwolter.com/projects/apple-health-export/
https://apps.apple.com/at/app/simple-health-export-csv/id1535380115
That is a very high rate
