Hi
Apple Watch does not support tracking endless / tethered swims as a specific activity type or recording distance for them.
To record the most accurate results for workouts on your Apple Watch, be sure to use the Workout app (or a third-party fitness app) and to choose the activity type that most closely matches your workout.
This enables your Apple Watch to use the most appropriate sensors and data sources when estimating results, including using tailored calorie calculations for certain workouts.
When tracking distance, the Open Water Swim workout type relies on GPS data, which is not relevant when remaining stationary. The Pool Swim activity type assumes that you are swimming laps in a pool, so would also not be appropriate.
To record workouts that are neither included within nor sufficiently similar to those in the main list, choose "Other" as the activity type.
For "Other" workouts, Apple Watch credits one minute of Exercise for each full minute of the workout and Active Calories at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher.
Distance is not recorded for Other workouts.
Keep in mind that water may prevent or reduce the accuracy of heart rate sensor performance during swimming workouts. For the best performance from the sensor, Apple suggests that you consider tightening your Apple Watch band before starting a workout and loosening it afterwards:
Wearing your Apple Watch - Apple Support
More information:
Work out with your Apple Watch - Apple Support
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Get the most accurate measurements using your Apple Watch - Apple Support
Swimming
If you would like to submit a feature request to Apple, then you can do so here:
Feedback - Apple Watch - Apple
