Swimming with Polar Loop 2 and M400?
Polar Loop 2 and M400 are water resistant and good for tracking activity when you swim, but they can't record heart rate in water. That is because the Bluetooth Smart used in both devices for transmitting data does not work in water.
Polar H7 heart rate sensor transmits data via Bluetooth Smart and via GymLink. GymLink does work in water, but Polar Loop 2 and M400 don't support it.