The problem is the direct contact to the water, a blanket with water running through it in a circle with a heat spreader or a different cooling device would solve that and would reduce the amount of water by a lot.
The blanket would be a bit heavier but by that it would double as a weighted blanket too.
Instead of a blanket the same could be built into the mattresses, not really a water bed but a bed with water cooling.
Devices, for example computers, get cooled like that.
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. 🫠
I'd imagine the effect was exasperated due to the pressure your feet are under when walking, whereas in a bath the weight is distributed and you're mostly buoyant. Its probably fine.
I actually didn't do much walking in them. It was a lazy Sunday afternoon/evening, with me mostly seated in an armchair and my legs put up, so the wet shoes didn't soak my furniture.
Personally, I even thought a bit of pressure might have stopped my skin from bloating quite as much.
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.
Besides all the potential medications you'd wake up panicked, in water (likely swallow more), and may in your panick fail to get out of whatever tub setup you have. It isn't common but people do drown in bathtubs (not just infants who've been left alone).
I think there may also be some medical consequences. I really don't know of there's long-term effects, but being in water constantly dehydrates you. It's the reason your skin gets wrinkly when you've been in the bath a while: your body thinks you're over-hydrated and tries to expel water through your skin.
Take that with a grain of salt; I read it somewhere years ago, and really have no idea of it's accurate, true, or whether it'd make a difference health-wise in any case.
It's actually a reflexive response by your body caused by the constriction of blood vessels in the skin, improving grip on wet surfaces. If you have nerve damage to the hands, feet, or related areas of the spine, the wrinkle reflex won't function.
I suspect our temperature regulation goes out of the window too as our bodies are used to be mainly in the air (not conducting, nor conserving, nor dense) and built a range of mechanisms to cool itself or maintain the livable temp to the main organs first. Sauna is a great example in how we survive hot air at temps nearing a boiling point of water by oversweating, while water makes this ineffective and a spilled coffee can cause severe burns. If there's a chance for the bathbeds, the reaction of the body to the temp should be considered too.
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
I'd never heard of this before. A fan with a little flat vent that shoots under your sheet. I don't have any interest in that, but learned something new.
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.