Well, in the spa I went to, there was a long list of instructions and guidelines posted right outside, telling people not to stay in too long, not to go in if they had such-and-such medical conditions, and so on, which would presumably help to limit both the likelihood of an accident and their liability if a customer failed to follow them. And there was an alarm call button where you could reach it if you collapsed on the floor. I assume they have somebody monitoring either by camera or wandering through every so often to see if anyone’s keeled over, although I didn’t see anyone doing so. And in a spa there are likely to be other people coming and going to raise the alarm if anyone does pass out.
This needs to be added:
While 212 F water will take the skin off your hand like a glove, 216 F air is perfectly bearable for a while. Ever reached into an oven to take something out and have 350 degree air spill over your arms? No problem, right? Now try that with the boiling pot of 212 degree water on top of the stove (please, don’t!). Water has a much higher specific heat capacity than moist air does. What this means in practice is that water has much more heat energy in it per gram compared to air. This is why a potato will cook much faster when boiled than when baked.
Heat energy and temperature are not the same thing.