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It is irrelevant whether they can or not.

The FDA and other health agencies have made it crystal clear that these devices are allowed to provide health guidance only and not be used for anything critical to life.

If someone bought the watch for this feature and it didn't work Apple would be in serious trouble.

It's not out yet, but I believe that it is not monitoring your heart rate continuously, because it would require the IR leds on the back to be on at all times, which would drain the battery.

Probably need some kind of sampling method, rather than continious.

Fitbit Charge HR works exactly that way - the IR leds on the back are constantly on....even when I take it off they keep on blinking. And it works for 4-5 days. Obviously it doesn't have a huge screen like the Apple Watch does, but it's certainly possible to be monitoring the heartrate constantly.

If the Android watches are anything to go by, absolutely not. The heart rate monitor on my Moto360 is often out by 50% or more.