Is it OK to wear an Apple Watch in a sauna?

Water resistance isn't a permanent condition and may diminish over time. Apple Watch can't be rechecked or resealed for water resistance. The following may affect the water resistance of your Apple Watch and should be avoided:

• Dropping Apple Watch or subjecting it to other impacts.
• Exposing Apple Watch to soap or soapy water.
• Exposing Apple Watch to perfume, solvents, detergent, acids or acidic foods, insect repellent, lotions, sunscreen, oil, or hair dye.
• Exposing Apple Watch to high velocity water, for example while water skiing.
• Wearing Apple Watch in the sauna or steam room.

Above from:

About Apple Watch water resistance - Apple Support