There’s no guarantee that The water resistance won’t fail. However, it is rated at 50 m water resistance, so it should be safe to use underwater, but with anything with soaps can deteriorate the water seals on the watch, Which you will want to make sure you rinse it off early when you’re through. Apple also recommends this off their website.
It shouldn’t be an issue, there are those who use their Apple Watches in hot tubs and don’t have problems. But again, its *not* a guarantee. The only thing I could also think of, is it steam can penetrate pass the water seals, that can be problematic.
I've worn my Series 2 watch into my hot tub countless times. Temps anywhere from 97 to 102 degrees and havent had an issue. As others have said, soap will degrade the seals, so I dont shower with my watch on.
Wouldn't expect any issue as long as it is short term. Two potential problems would be the seals being damaged by soap/detergent long term (not likely, and you're also not going to have it in the bath for days continuously), or pressure. This is also partially how the water certification is tested, so hot tub jets could be a potential great risk, but I'd still say it's very unlikely to cause damage.