Open water swimming

It won’t. You need to post your feedback to Apple direct.

Get better results

• Your Apple Watch estimates your measurements during workouts based on the information that you entered during setup. Here's how each workout is measured: 
  • 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.

https://www.apple.com/feedback/

The open swim uses gps and depending on what strokes you use might not be accurate in open water