If you have heavily tattooed wrists, you might want to reconsider getting the Apple Watch. As several people have reported, the heart rate sensor on the back of the watch has trouble doing its job if you have tattoos on your skin.
Apple’s heart rate monitor counts heart rate by pulsing green LEDs through the skin several hundred times per second. Any obstruction between the sensor and the skin can adversely affect the performance of the sensor.
As you can see in the video below, the sensor has a hard time detecting the skin on tattooed wrists. This means it can stop counting your steps or other activities if it doesn’t get a proper read. But by far the most annoying side effect of this is that the watch will constantly ask you for your password, whereas normally it would just detect that it is worn and bypass the lockscreen.
No known solution exists for now and I doubt Apple would be able to fix this without significantly altering the functionality of the sensor. And considering it’s not going to affect a majority of the people, it would be less inclined to.