Swimming..

The VA3 by itself is not designed for open water swimming. It can be used for pool swimming, but the heart rate from the oHRM will be worthless. The Garmin HRM-SWIM chest strap is ANT+ compatible and should work fine the VA3 for swimming. Double check with Garmin as I'm not 100% sure how well the VA3 will receive signals underwater from a chest strap.

None of these devices will record HR data in the water from the wrist whether open water or pool. You need an external strap. Period!

As for HR in the water from HRM-swim and HRM-Tri, the data is collected on the strap during the swim, then transferred to the watch afterwards, not during the swim.

@philipshambrook: As I read in DC Rainmaker, VVA3 is not compatible with HRM-swim. I don't know it's a limited by hardware or something else. Can you give more information ?

Anyway, this morning, I do Lap swimming with Data Field named Dozen Swim. It does show instant HR data and seems to be correct. But there's no HR data recorded in Garmin Connect.

And there won't be because HR is disabled in the swim as far as Garmin is concerned. You might see some HR data when you are stopped. That is not recording data during the activity for analysis afterwards.

None of these devices will record HR data in the water from the wrist whether open water or pool. You need an external strap. Period!

As for HR in the water from HRM-swim and HRM-Tri, the data is collected on the strap during the swim, then transferred to the watch afterwards, not during the swim.

I don't believe the data is collected during the swim and then transferred afterwards is correct. I wear a chest strap for every workout I do, though never for a swim. Anyway, my chest strap is synced and I can see my current HR on my devices while working out. There is no need to transfer the data afterwards. Fanvantan, you are correct. DC Rainmaker does state the HRM-SWIM is not compatible with the VA3.

Thanks for the clarification on HRM straps- was thinking about those but decided it's overkill. I'm still confused though - my VA3 is definitely recording HR data (accurate or not) during my swims - so the monitor is doing something. I'm going to have a look at the FIT files when I get chance to see what's going on. I don't take any breaks in a 30 min swim session so I doubt that's the reason - I can't show it on the screenshot but if I hover my mouse over the HR graph I can see detailed recordings.

Fanvantan, you are correct. DC Rainmaker does state the HRM-SWIM is not compatible with the VA3.

However, this site indicates they are compatible: https://www.thisisant.com/directory/hrm-swim
So, either jury is still out or something changed.
It would be great to get an official Garmin answer.

The HRM-Swim can be connected to the VA3 but only as a regular chest strap. It won't be able to upload stored HR data to the watch.

I would like to echo what tmk2 has said as the correct answer. The vivoactive 3 is not compatible with swimming HR from a strap or the optical sensor.