can i wear my iwatch in the 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

Even if you are in a dry sauna (no rocks to wet) the heat will be well above the recommended range of the Watch. This is not good for the battery in your Watch and you will end up significantly decreasing the life of it. Same goes for your iPhone.