I have been using mine in a Sauna and steam room for 3 months. 15 mins every other day at 90C maximum. No problems except that the screen saver may be beginning to peel. I suggest using a suitable watchface to show temperature and heart rate.
Using Vivoactive 3 in sauna almost every week, tried in many saunas, even in extremely hot ones - no problems so far. I just cooling it down every 3-5 minutes by putting and holdin palm until you feel watch cooled down(not feeling it to be hot).