If my phone dies . .

Also:

Features that can be used on your watch without your iPhone being connected may change under watchOS 2.

watchOS 2 is a major operating system / software update for Apple Watch. After being released on 16 September, it can be installed by following the instructions here:

Update the software on your Apple Watch - Apple Support

Hi

If your iPhone is not connected (including if it has run out of battery charge), you can still use Apple Watch to:

- Play music from a synced playlist on Apple Watch.

- Use the watch, alarms, timers, and the stopwatch.

- Keep track of your activity (stand, move, exercise) with the Activity Activity icon app.

- Track workouts.

- Display photos from synced photo albums.

- Use Apple Pay to make purchases.

More info: Use Apple Watch without its paired iPhone

You will need an iPhone running iOS 9 to upgrade to watch OS 2 and if your watch ever had a serious software problem you would have to re-pair it with an iPhone.