power reserve vs airplane mode
What is the difference between switching the I-watch into power reserve vs putting the watch in airplane mode?
Apple Watch 42mm, watchOS 1.0.1
In Airplane Mode, Apple Watch disables both Bluetooth and Wi-Fi connectivity, so it can't communicate with your iPhone or use any functions that rely on that connection (eg calls, messages, playing music from your iPhone).
You can still use features that don't rely on the iPhone connection - eg tell the time, play music from a synced playlist on Apple Watch, alarms, timers, stopwatch, keep track of your activity (stand, move, exercise) with the Activity app, track workouts (without GPS), display photos from synced photo albums, use Apple Pay.
https://help.apple.com/watch/#/apd6e1809c9e
In Power Reserve mode, all that you can do is tell the time (by pressing the side button once). This is designed to keep this function available for as long as possible (Apple Watch automatically switches into Power Reserve mode when the battery gets very low).