If I let someone use my Apple Watch can I have it turned off if they won’t give it back to me?

Letting someone use your Watch while still logged in as you will give them access to your data. I would seriously rethink this if you are already concerned about them possibly not returning the device. Consider the value of the watch. I would not loan anyone that I am concerned about returning anything something that costs as much as an Apple Watch.

If you wiped the Watch completely, then you would not be able to use Find My iPhone to locate it. If you didn't, they would have difficulty pairing the watch with their device. Just wanted to share that, since you did not provide a great deal of info on how you intended to share, and I wanted to be sure you understood the privacy issue. Good luck with your loan.

If you didn't give them your Passcode and you have on "Find My iPhone" app....the answer is yes.