Yes, you can.
By default, it does leave bluetooth on so as to not to lose connection from the paired iPhone. Thoough, you can change the behaviour by going into Watch App > General > Airplane Mode > turning off the toggle for bluetooth. If you turn off Bluetooth while you're in Airplane Mode, the watch will remember the next time you turn on Airplane Mode.
You can use Airplane Mode to turn off the wireless features on your device while you're flying in an airplane.
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