To be water cooled we would need some kind of radiator, I guess moving blood through your extremities could work but seems risky from an evolutionary perspective. Like if you’re hot from running from a predator your hands and feet would be flush with blood.

Also I think evaporative cooling could help you maintain a body temp of 38 when the ambient temp is somewhat warmer than that like 45 or so. I don’t think a radiator could do that.

Sorry to get all practical in this thread about not needing to poop. I was just kinda thinking out loud.