Apple watch not accurately measuring swimming thought workout app

Does your watch detect your breast stroke in the workout details after you stopped your workout?

Maybe it helps to lift your watch out of the water after you finished one lane and start a new one.

Go for a swim with Apple Watch - Apple Support

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Swimming

With Apple Watch Series 2 or later, you can choose Pool Swim or Open Water Swim.* When the workout starts, the screen automatically locks to prevent accidental taps from water droplets. Learn how to swim with your Apple Watch and view your workout history, including auto sets and splits

Here's how your Apple Watch measures each workout:

• Pool Swim: When you start a workout, be sure to accurately set the pool length to help your Apple Watch measure laps and distance. GPS isn't used during a Pool Swim, and water might prevent a heart-rate measurement, but calories, laps, and distance will still be tracked using the built-in accelerometer.
• Open Water Swim: GPS will only provide distance when you do a freestyle stroke. Water might prevent a heart-rate measurement, but calories will still be tracked using the built-in accelerometer.

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copied from: Workout types on Apple Watch - Apple Support

Did you use Freestyle or Breaststroke as a technique?

You will get the best results if using Freestyle, because the watch needs to be above water to register the distance and count your strokes during your exercise.