Operating temperatures are between 0-35°C and can be stored from -20-45°C. I'd probably leave it off if you're going to be in the hot tub for a long time, or make sure you dont submerge it for long periods of time. https://support.apple.com/en-gb/HT204508
i wouldn't chance it in the hot tub. heck i don't even wear my watch when i shower. i also have an insulin pump that i remove when i shower, even though it's waterproof @ 12ft.
I've taken my series 0 into the hot tub. 90% of the time my arms are out of the water. No issues with it. Hot tub isn't going to be much hotter than 95 degrees. I typically keep mine set at 97-99. I shower with my series 0 ever single day, have been for 17 months. 0 issues.