In short, balnotherapy (bathing and exercising in mineral water)
does not likely:
- provide essential minerals through dermal absorption
- improve symptoms of rheumatoid arthritis, other rheumatic conditions, osteoarthritis or fibromyalgia better than hydrotherapy (immersing in plain water)...
...but may help in dealing with stress associated with rheumatic conditions.
In certain European countries with a lot of natural spas, it's a habit that a doctor prescribes balnotherapy to a rheumatic or orthopedic patient as a form of physiotherapy; it's meant more as a stress relief than actual cure.
Can minerals be absorbed through the skin?
I did this research about dermal absorption of minerals once and the only thing I found is a small study in which they observed that bathing in Epsom salt (magnesium sulfate) was associated with increased blood magnesium levels (epsomsaltcouncil.org).
According to a review in Nutrients, 2017 other forms of magnesium, such as magnesium chloride, are not likely absorbed through the skin.
I haven't found any evidence that any other essential mineral would be absorbed through the skin in meaningful amounts.
Does balnotherapy has any beneficial health effects?
Balneotherapy (or spa therapy) for rheumatoid arthritis. An abridged version of Cochrane Systematic Review (European Journal of Physical and Rehabilitation Medicine, 2015):
Overall evidence is insufficient to show that balneotherapy is more
effective than no treatment; that one type of bath is more effective
than another or that one type of bath is more effective than exercise
or relaxation therapy.
Evidence-based hydro- and balneotherapy in Hungary—a systematic review and meta-analysis, International journal of biometeorology, 2014):
Based on the results, we conclude that balneotherapy with Hungarian
thermal-mineral waters is an effective remedy for lower back pain, as
well as for knee and hand osteoarthritis.
Treatment of Osteoarthritis of the Knee: An Update Review (AHRQ, 2017):
Intra-articular platelet-rich plasma, balneotherapy, and whole body
vibration show medium-term benefits.
Therapeutic benefit of balneotherapy and hydrotherapy in the management of fibromyalgia syndrome: a qualitative systematic review and meta-analysis of randomized controlled trials (Arthritis Research & Therapy, 2014):
High-quality studies with larger sample sizes are needed to confirm
the therapeutic benefit of BT and HT, with focus on long-term results
and maintenance of the beneficial effects. (BT = balnotherapy; HT =
Balneotherapy, Immune System, and Stress Response: A Hormetic Strategy? (International Journal of Molecular Sciences, 2018):
Balneotherapy is an effective complementary approach in the management
of several low-grade inflammation- and stress-related pathologies,
especially rheumatic and metabolic conditions.