My parents used to have a waterbed years ago. Temperature wise it was really great. Turn the temp up in winter for a cozy bed and down in summer for a cool sleep.

When I was a teenager at one point I had a waterbed and left the heat off. While not the same thing, it was effective at stealing body heat. I have no idea whether it was a good idea though

A few months ago, I bought some fake/vegan leather shoes and they didn’t quite fit. So, I figured, I’d try the old leather shoe trick, where you fill them with water, put them on and let them dry+shrink to fit your feet.

And it actually kind of worked. But I had decided to just leave them on for basically the rest of the day. And that wasn’t a great idea.

My foot soles had soaked with water and gotten wrinkles that carved about a centimeter deep. Even after taking the shoes off, I was in pain for a few hours. It felt like the soaked skin was constantly pulling on my flesh, because their shapes didn’t fit together anymore.

So, yeah, I imagine, the same would happen, if you bathed for 8 hours. You could somewhat mitigate it, by covering your whole body in vaseline or some other form of fat.
I guess, you could also try bathing in rapeseed oil or such directly…? I’m not actually recommending this, though. 🫠

Things growing in the water would be an issue. And highly treated water will damage your skin. There’d also be a chance of drowning every night. You could mitigate with a dry suit but that kinda ruins the point.

Get an under sheet fan.