As far as the physics of circulation, I suspect immersion would relieve stress on the circulatory system. Underwater, the pressure change with elevation is the same both inside and outside our bodies, which means the circulatory system doesn't have to compensate for orientation.
However, based on geometry I estimate laying flat would relieve about 90% as much orthostatic stress as being immersed, and most of us recognize laying flat is less tiring.
He acknowledges that equivalent or greater increases in o2 partial pressure don't cause the benefit.
Next he discusses the mammalian dive reflex, which is weak in adult humans and associated with cold water. He's wearing a t-shirt underwater, which means the water is very warm, certainly over 25c. His mask covers his nose and he is breathing 0% humidity gas, so the nerves inside the nose that detect immersion are probably not being activated.
He continues by speculating about vagus activation and parasympathetic shift...
I'm not buying any of that, but if others have noticed something similar with diving, maybe I'll have to dust my gear off. It's also plausible this person has a mild or relapsing remitting condition and they feel better on vacation.