The solar panel of Solar Paper are is water resistant. ( IPcode 64) However, an USB part is susceptible to corrosion so make sure to dry it well after exposure to water. If the panels have been exposed to seawater or water with salinity, rinsed the panels with clean water and dry.
Water resistant level IP64. Here is a link about IPcode wikipedia.
"First digit, 6, means: Dust tight No ingress of dust; complete protection against contact (dust tight). Second degin, 4, means: Splashing of water Water splashing against the enclosure from any direction shall have no harmful effect.“
Immersion in water will also be okay(not strongly recommended ) but make sure to dry it thoroughly.