Apple Watch keeps pausing during workouts

I have the same issue and what I found that actually does work is to turn off the wrist detection. The purpose of wrist detection is to automatically lock the watch when removed from your wrist so you will just need to get in the habit of manually locking it when you take it off. I along with everyone else was hoping that WatchOS3 was going to fix this issue but I guess we'll have to wait a bit longer.

Workouts may also be paused or ended early due to accidental interactions with your watch's screen (including by clothing). To help prevent this, you can lock the screen:

• At any time after starting a workout, press firmly on the display and tap on Lock.
• When you are ready to end the workout, press firmly again and first tap on Unlock.

If you have not yet done so, it may also help to update your iPhone and Apple Watch to the latest software, starting with your iPhone:

• Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
• Update the software on your Apple Watch - Apple Support

Hi

Check that Wrist Detection is turned on:

- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection.

If Passcode is enabled, check also that you are unlocking your watch after placing it on your wrist.

Tracking can be affected if your watch loses contact with your skin, causing it to believe that it has been removed from your wrist. This would cause the Workout app to pause automatically.

A loose fitting band can trigger this by allowing your watch to lift from your wrist and/or by allowing clothing (for example, the end of a sleeve) to slip beneath the back of the watch. Sensor performance can also be affected by temporary or permanent changes to your skin, such as some wrist tattoos.

During exercise, Apple Watch should be worn snugly on the top of your wrist. Apple suggests that you consider tightening your band before workouts and loosening it again afterwards. Better contact with your wrist may also improve heart rate sensor performance during your workout.

More information:

Use the Workout app on your Apple Watch - Apple Support

Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support

Make sure your watch fits correctly > Wearing Apple Watch - Apple Support