In times before pharmacologists described efficient drugs, balneotherapy was the only therapy applied.
Physicians would propose relaxation as well in a calm environment, and different massage techniques in dedicated centers.
This approach was fully efficient in most cases.
Already Romans in Ancient Times would search for thermal sources wherever they would build bathing facilities for local population.
During my studies in the 70ties a professor of Medicine questioned skin pressure receptors as an explanation for efficiency of baths in lowering blood pressure.
Reality is much more trivial. Water used is usually several degrees below body temperature, and thus elicits vasoconstriction, which in turn frees a volume of liquid, which is taken up by the kidneys and excreted.
In an equivalent way, rest in recumbent position lowers blood pressure.
When you are upright, more circulating volume is mandatory to maintain pressure.
When lying down, the kidneys would excrete volume excess progressively.