With a waterproof rating of Rolex Submariner or higher, submersion in water is not a problem at all. For reference, if moisture appears on the inside of the glass, it means that there is a problem with the current waterproof performance and you should send it in for repair right away.
Regardless of the water resistance issue, it is not a good idea to enter a hot sauna while wearing a watch. First of all, the body can withstand temperatures as high as 60-70 degrees, but if the metal gets hot, it can damage the skin. The oil inside the watch also has an effect. Just as cooking oil becomes softer like water as the temperature rises, oil in movement parts can move out of its place and lead to oil diffusion. We recommend that you store your valuables carefully before going to the sauna.
Tom
VIVER Chief Engineer