I thought a friend of mine was using the Apple watch for ECG a couple of years ago but I think that was an older version with something extra from here
https://store.alivecor.com/I don't think they sell the same the same thing he was using anymore, probably because the new watch has it built in.
I have a handheld ECG somewhere that you could borrow if I can find it and it still works but it isn't very useful for exercise induced problems. You have to be very still and relaxed to get a good signal.