Just like other wrist-based heart rate monitors, I've found the Apple Watch will be within 10% of a good chest strap HR monitor (like the POLAR H10), but only

Just like other wrist-based heart rate monitors, I've found the Apple Watch will be within 10% of a good chest strap HR monitor (like the POLAR H10), but only during rest. During high intensity interval training, the AW (and Garmin) can struggle. For me, it'll regularly read grossly under or over (like, think you're at 90 when your HR is actually 140, or 180 when I'm at 150). Depending on your veins, your skin color, and your activity, wrist HR might be fine for you. If you want accurate HR in most conditions, though, it seems that you have to resort to a chest strap.