Heart rate not tracked during activity

Hello everyone,

I recently bought this Garmin Instinct watch that is great but which doesn't seem to track my heart rate if I start an activity.

Let's say I go swimming for 1 hour: I get every data of my performance, strokes/100m, t/100m,... but my heart rate graph is empty during that period of time. However, if I do jumping rope and don't ask to record any activity, my heart rate will be tracked precisely; there is no dysfunction for resting heart rate tracking.

Even after going through the settings I cannot find how to enable this function.

Is this something normal that I did not understand? Or is there a way to enable this?

Many thanks!