Apple says;
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
Wrist detection off;
Turning off wrist detection affects these Apple Watch features:
When you use Apple Pay on your Apple Watch, you’ll be prompted to enter your passcode when you double-click the side button to authorize the payment.
Some Activity measurements are unavailable.
Heart rate tracking and notifications are turned off.
Apple Watch will no longer automatically lock and unlock.
Apple Watch SE and Apple Watch Series 4 and later won't automatically make an emergency call even after it has detected a hard impact fall.