It's only appropriate that my first post back here after 9+ years on the other side of the Rule book should be on a Rules topic.
Rule 4.3a says, in part:
A player may use equipment to help his or her play during a round, except that a player must not create a potential advantage by:
Using equipment (other than a club or a ball) that artificially eliminates or reduces the need for a skill or judgment that is essential to the challenge of the game,
In order to be in breach of the Rule, you must be using the watch in a way that breaches this. Two examples:
1) I walk up a steep hill and wonder how that affected my heart rate so I look at it out of curiosity.
2) i walk up a steep hill and want to make sure my heart rate has come back down before I play my next stroke.
#1 is not a breach, #2 is. In the first example I'm not using it to reduce the need for judgment, but in the second I am. This is like other examples in the Rule such as listening to music just to pass the time vs. listening to help my tempo.
It's nice to be back.