There are several factors that influence the ability of the heart rate sensor (of both devices) to accurately monitor heart rate data. These include skin perfusion (which is different from person to person and impacted by temperature), fit, the presence of tattoos, water (swimming) and motion (rhythmic movements are better).  If you think your Apple Watch or WHOOP isn’t correctly recording heart rate data, make sure the strap is tight enough around your wrist and that the sensor isn’t directly above your wrist bone (it should be about one inch away from it).  If adjusting the tightness of the strap and the location of the sensor doesn’t work, consider wearing the device on your other wrist or using an arm or bicep strap.