How to Use Interval with Apple Music:

You should have paired the Interval with the Apple Watch - show in the video above.
Now - go to your Watch app on your iPhone - and find and open Music app (below).
Then - make sure you have some music synced to your watch. Select the +Add music to manually add a song / album. Your Apple Watch MUST BE CONNECTED TO THE CHARGER before music is synced.
swimming apple watch
Once the music is synced, you can even turn off your iPhone.
You must start the music from the watch music app -
swimming with music
 
Then - Select the music and press play. The Watch will now try to find the paired Interval.
Make sure the Interval is powered on. Music should now start to play from Interval speakers.
apple watch swimming music