They have a free app to connect to your iPad. Then you can monitor the heart rates consistently and you can intervene immediately if the swimmers don’t go hard enough, or too hard.
You have to do max heart rate test though, from time to time to update the settings.
https://www.hiddenbrains.com/blog/wearable-app-development-next-phase-of-change.html
https://newatlas.com/sports/athlos-live-swim-coaching-earbuds/
Andy Muller posted that he uses the Polar Verity Sense
https://www.polar.com/au-en/products/accessories/polar-verity-sense/
They have a free app to connect to your iPad. Then you can monitor the heart rates consistently and you can intervene immediately if the swimmers don’t go hard enough, or too hard.
You have to do max heart rate test though, from time to time to update the settings.
We had great success with it.