The Institute of Physical performance at Norwegian School of Sport Sciences has published a validation study on Polar heart rate technologies in swimming. In this study twenty-six well-trained competitive swimmers performed a regular training session including different swimming intensities. During the training session, the swimmers wore a Polar H10 heart rate sensor with Polar Pro strap (H10) underneath the swimsuit, a Polar OH1 optical HR sensor (OH1) underneath the swimming cap at the temple, and a sports watch with optical HR sensor, Polar M600 smartwatch (M600) on the wrist. Based on the results, no difference in minimum, maximum and mean heart rates between H10 and OH1 was found, but the maximum and mean heart rates obtained by the M600 were significantly (about 10 bpm) lower than with other devices. Researchers concluded that the Polar OH1 optical HR sensor is a valid tool to monitor heart rate of different intensities during swimming whereas the Polar M600 smart watch as a wrist worn device is less accurate.
Olstad and Zinner. Validation of the Polar OH1 and M600 optical heart rate sensors during front crawl swim training. PloS ONE 15(4):e0231522.https://doi.org/10.1371/journal.pone.0231522