Using a device that gathers heart rate data via a wrist-based optical sensor during swimming can present some unique challenges when trying to get an accurate reading.
Optical heart rate sensors work by using a green light that can measure small changes or pulsations in blood volume in the capillary layer just below the epidermis (outermost of the three layers that make up the skin) as blood flows through the wrist area.
When you are swimming, the very act of taking a stroke can affect the level of blood volume in the wrist area and chilly water can reduce the amount of blood in the tissue on the back of the wrist. This can cause the optical sensor to have problems detecting pulsations.
The drag of the water can also pull on the watch creating a gap between the optical sensor and the skin - allowing ambient light to enter and potentially skew readings. Another possibility is that water droplets can get between your skin and the sensor, causing the light to refract and also skew the reading.
To help combat these issues, there are a few things you can do.
1) Warm up a bit before starting to record your swim to get your blood flowing.
2) Make sure you are properly wearing the watch. This means that the watch is snug so that the sensor can remain in contact with your skin and the watch should not be sliding up and down your arm. Do not over-tighten the band, which could restrict blood flow. Wear the watch just above your wrist bone, so that the sensor has access to an area of your wrist where it can more easily sense pulsation in your capillaries.