YOON, K. S., HUANG, X. Z., YOON, Y. S., KIM, S. K., SONG, S. B., CHANG, B. S., KIM, D. H. & LEE, K. J. (2011). Histological study on the effect of electrolyzed reduced water-bathing on UVB radiation-induced skin injury in hairless mice. Biological and Pharmaceutical Bulletin 34, 1671-7.

One group of researchers gave hairless mice sunburns. They bathed one group in tap water and the other in hydrogen water.
The skin of the mice bathed in hydrogen water showed significantly lower indices of skin injury and lower inflammatory cytokine levels.

IGNACIO, R. M., YOON, Y. S., SAJO, M. E. J., KIM, C. S., KIM, D. H., KIM, S. K. & LEE, K. J. (2013). The balneotherapy effect of hydrogen reduced water on UVB-mediated skin injury in hairless mice. Molecular & Cellular Toxicology 9, 15-21.

Another group of researchers performed a similar study and found that bathing mice in hydrogen water reduced the level of skin damage, increased activity of the antioxidant glutathione peroxidase, lowered inflammatory cytokines, and prevented ultrastructure changes of the skin, suggesting hydrogen water can protect against UV-induced skin cell damage.

KATO, S., SAITOH, Y., IWAI, K. & MIWA, N. (2012). Hydrogen-rich electrolyzed warm water represses wrinkle formation against UVA ray together with type-I collagen production and oxidative-stress diminishment in fibroblasts and cell-injury prevention in keratinocytes. J Photochem Photobiol B 106, 24-33.

This was further validated in another article where hydrogen water was shown to be very beneficial for UV induced skin cell damage.
The study showed that Type-1 collagen was synthesized about two-fold more in the cells treated with hydrogen water. It also prevented DNA damage, cell death, and decreased levels of intracellular free radicals. The study also reveals that in human subjects, bathing in hydrogen water for three months significantly improved wrinkles in the skin. The authors concluded that hydrogen water might serve as a daily skin care to repress UVA-induced skin damage by scavenging free radicals and promoting type-1 collagen synthesis.

1) YU, W. T., CHIU, Y. C., LEE, C. H., YOSHIOKA, T. & YU, H. S. (2013). Hydrogen-enriched water restoration of impaired calcium propagation by arsenic in primary keratinocytes. Journal of Asian Earth Sciences 77, 342-348.

2).OHSAWA, I., ISHIKAWA, M., TAKAHASHI, K., WATANABE, M., NISHIMAKI, K., YAMAGATA, K., KATSURA, K., KATAYAMA, Y., ASOH, S. & OHTA, S. (2007). Hydrogen acts as a therapeutic antioxidant by selectively reducing cytotoxic oxygen radicals. Nat Med 13, 688-694.

3).ITOH, T., HAMADA, N., TERAZAWA, R., ITO, M., OHNO, K., ICHIHARA, M. & NOZAWA, Y. (2011). Molecular hydrogen inhibits lipopolysaccharide/interferon gamma-induced nitric oxide production through modulation of signal transduction in macrophages. Biochemical and Biophysical Research Communications 411, 143-9.
Hydrogen water was also seen to prevent arsenic-impaired calcium signaling (which is involved in skin cancer) in keratinocytes (predominate cell type in skin) through both its antioxidant and non-antioxidant cell signaling effects.
In other words, hydrogen not only exerts protection and benefits on the skin because it’s an antioxidant, but also as a cell signaling molecule.