james wrote:
I want to turn off my water main to repair an outside faucet. Do I have
to shutdown the water heater first?

You should turn off the heater.
Under normal circumstances, the water stays in the tank and is not a
problem.
But what about abnormal circumstances? Like you get distracted, or
hurt...and the tank drains.

It takes two seconds to flip the switch to off. Why not take the
precaution???

And that goes for any kind of repair. Don't skip simple steps
that make the process safer or less vulnerable to screw-ups.

And before the villages light the torches to come after me, let me
say...
The fact that the statistical probability of having a problem is near zero
is small comfort when it actually happens to you.

And after I turn the water back on,
do I have to do something to "reboot" the water heater?
no, but read the manual if you're unsure.