Does H10 measure heart rate when swimming?
Yes, Polar H10 HR sensor is a good choice for swimming. You can record a heart rate file in the sensor memory or use the sensor with a watch having 5 kHz GymLink connection during swimming.
When using the built-in sensor memory, connect H10 to the Polar Beat app, and tap Save HR with sensor. See in more detail here.
When using older Polar watches having 5 kHz GymLink connection, remember to check that you have enabled H10 GymLink option in the Polar Beat app settings as Bluetooth doesn't transmit data in the water. See in more detail here.
It's good to notice that sea and pool water are very conductive which may prevent the chest strap detect heart rate impulses in water.