Balneotherapy- 

The treatment of disease by bathing or immersion in mineral water or mineral-containing mud, a traditional medicine technique practiced at spas, and part of several traditional folk medicine treatments.  Balneotherapy may involve hot or cold water, massage through moving water, relaxation, or stimulation.  It is distinct from hydrotherapy, but there are some overlaps.  Balneotherapy has been used to treat many diseases including arthritis, fibromyalgia and for relaxation.  Another type of balneotherapy uses medicinal clays and is known as fangotherpy.  There is one quality RCT showing balneotherapy was superior to paroxetine (Paxil) for generalized anxiety disorder (Adv Ther 2018 Mar 5. Doi: 10.1007/s12325-018-0680-6).    There is some alpha level evidence of effectiveness.

      There are mixed results on cortisol levels with balneotherapy (Int J Biometerol 2018 Feb 18:doi:10.0007/s00484-018-1504-8).  Baleotherapy is clinically effective in relieving pain and stiffness in osteoarthritis of the knee, and improving function in a meta-analysis (Clin Rheumatol 2017 Aug;36(8):1839-47).   There is insufficient evidence to recommend balneotherapy for the treatment of rheumatoid arthritis (Eur J Phys Rehabil Med 2015 Dec;51(6):833-47).  For fibromyalgia syndrome, balneotherapy results in a small reduction in pain and large quantity of improvement of Quality of life) (Arthritis Res Ther, 2014 Jul7;R 141).  Balneotherapy results in an improvement short term for knee osteoarthritis (Clin Rheumatol 2009 May;28(5):501-7)