Normally you can see the heart rate information for running/hiking/skiing. But it’s not the case by default for pool swimming with Workout app.

On Apple Watch, Workout, touch the “…” button on Pool Swim(OPEN GOAL) section, click the pencil button on the top-right corner of “Open” section, Workout Views, Edit Views, enable the “Include” of “METRICS 2” for heart rate information and “HEART RATE ZONES” if needed. You can also “Reorder” the views, the button for it at the bottom of the page.

Reference:
* Apple: Use the Workout app on your Apple Watch
* Polar: HOW TO CALCULATE YOUR MAXIMUM HEART RATE
* Polar: HEART RATE ZONES | THE BASICS