The very first Apple Watch was released back in April 2015, and by 2016, it had become the best-selling smartwatch in the world.
You cannot wear an Apple Watch in a sauna or steam room. While Apple states that their newer generations of Apple Watch are water-resistant up to 50 meters (165 feet), they themselves claim that the environment in a sauna or steam room can cause damage to the watch, and recommend against it.
Waterproofing, whether speaking of watches or houses, is done to make an object impervious to water.
The biggest difference between waterproof and water resistance is the amount of water an object can withstand. If a watch, item, or clothing claims to be water-resistant, you have to expect limitations.
As the name would suggest, a steam room is heated by steam from boiling water. The closed room is usually kept as hot as 43°C (110°F).
Where a steam room uses humidity to heat the room, most saunas use a dryer heat. Though there is some moisture in the air, saunas typically use electric radiators or even infrared technology.