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.