The Vivoactive 3 and Forerunner 645, in particular, do NOT support ‘Open Water Swim’ though they do support ‘Pool Swim’.
This distinction and restriction drives folks nuts, so, I decided to get to the bottom of it.
It has all to do with GPS/Glonass support — or lack thereof IN WATER.
GPS does not work very well when the sensor is submerged in water. THAT IS THE CRUX OF THE ISSUE.
If you just want a GPS distance and track while you swim in open water you could try putting your watch into running mode and having it attached to a swim cap. That is supposed to work.
‘Pool Swim’, by definition, means that GPS is DISABLED. Pool swim data is measured using the ACCELEROMETER. Got that? No GPS … just accelerometer.
So, how do the more expensive watches like the Garmin Forerunner 935 and Fenix 5 support open water swimming? They use the GYROSCOPE.
With the gyroscope you only need occasional GPS fixes. The gyroscopes fills in the gaps between the GPS readings. The Vivoactive 3 does NOT have a gyroscope. Hence why open water swim is not available.
But, alas, a gyroscope alone is also NOT enough. For open water swim you ALSO need a powerful enough CPU to continually calculate the data. Got that?
Hence why open water swim is not supported on the Forerunner 645 though it has a gyroscope. The CPU in it is not as powerful as that in a FR935. Got that.
So, that is the deal.
To get open water swim support you need a gyroscope and a big enough CPU. Without that you only get pool swim which only uses the accelerometer.
You can TRY and use running or pool swim in open water — though the results will be mixed and most likely inaccurate.
So, that is the story. Now you know.