There are special seam sealers and seam tapes sold in camping stores. I echo the folks recommending something like a Lock and Lock or Pelican sturdy plastic container for protection of valuables, then making a bag to hold them. I used to scuba and no matter how careful I was something eventually got wet through leakage. You can make small tightly woven cloth bags of silica (from drying flowers kits) or reuse the silica bead packets that come with new shoes and purses to keep the humidity down so you won't get rust or molding in sealed containers. Just velcro them into the container and replace them frequently. This works even in a home safe.