Apple Watch SE compatibility with Garmin HRM-Swim

In theory, you can pair a chest strap.

Get the most accurate measurements using your Apple Watch - Apple Support

In practice, the information I'm seeing is mixed. There appear to be a lot of YouTube videos telling you how to pair the two devices. I also found this in the Garmin Forums:

https://forums.garmin.com/sports-fitness/running-multisport/f/accessories-sensors/332668/would-it-be-possible-to-use-garmin-hrm-sensors-without-a-garmin-watch-using-an-app

It's about a year old but it might get you started.

The Apple Watch is made to be worn on your arm, hence the name. I don’t suggest using the Apple Watch with third party accessories.