I've been looking at the KardiaMobile but the reviews on Amazon look pretty bad. How well does the Apple Watch perform since it's just on a wrist?

I honestly don't have the information to answer that.

I do know the single-lead EKG has some limitations, but they may not be relevant to your dad's particular scenario. Worth a conversation with the doctor, I suspect.

Yeah the real tricky thing is how well they do. We originally thought he had bradycardia because his heart rate as measured by his heart pressure cuff showed it in the high 30s. However once he got an EKG, it became clear that his heart rate was normal but he was afib and all the irregularity wasn't being picked up properly by the BP cuff monitor. I'm guessing that something like the apple watch is accurate enough to inform someone previously healthy that something is now wrong, but wouldn't be accurate enough to tell someone who is unhealthy if their condition is worsening.

The Apple Watch's EKG is definitely good enough for diagnosing bradycardia - you can see very clear QRS complexes showing up on it.

(I would not necessarily trust the 24/7 LED-based rate monitoring.)

Yeah, but brachycardia wasn't the problem and that's easy to diagnose. What I want to know is if it is good enough to know that it wasn't actually brachycardia but was instead afibrillation with weak parts of the cardiac cycle.