Certain exercises produce inaccurate heart rate readings on the Apple Watch
One P90X user had a Polar heart rate strap on, with the Polar watch on one wrist, and the Apple Watch on the other. The Polar device showed a heart rate above 160 during the exercise, while the Apple Watch showed a rate of 70-80bpm. Apple has sent replacement watches to users with this issue, but that has not cleared up the heart rate inaccuracies.
The Apple Watch uses a method called photoplethysmography that measures blood flow in the wrist, to determine a user's heart rate. Apple's own website lists certain activities that could produce inconsistent results with this process. Activities like tennis and boxing will produce inconsistent and inaccurate heart rate readings on the Apple Watch, because those feature movements that are "irregular." Not surprisingly, exercises that have a certain rhythm, like cycling, or running, will work better with the watch. Still, some users who are doing exercises like running and walking outdoors, both of which are recommended by Apple's workout app, are having the same issues with their Apple Watch.
No word yet from Apple about this. Since they have supposedly sent out replacement watches to some, they are obviously aware of the problem. In addition, you might recall that earlier this year, it was discovered that Apple Watch users with a tattoo on their wrist could not get a heart rate reading from the watch since the ink blocks the sensors on the device. Some claim that moving the watch up toward the elbow will produce a more accurate reading, but Apple does not recommend doing this since it built the watch specifically to be used on the wrist.
source: Apple, Reddit via AppleInsider