Sometimes to win a war or battle a nation must conduct amphibious operations and such, that require soldiers to enter water. Experiencing this, "X" nation decided to create a branch of marines/naval fusiliers in their army/navy.
However, they've run into an obstacle: water is incredibly harmful to military equipment and these soldiers are required to enter bodies of water all the time.
How could the military high command work around this? I fear the equipment that will need the most protection will be:
Gunpowder & Muskets
Bow/Crossbow strings (yes, for a number of reasons, mostly involving fantasy, they're still being used)
Metal armor (same as n. 2)
Would there be any way with the available technology to protect this equipment from water or, if possible (but very unlikely), make it water proof?
P.S. Having the marines/naval fusiliers and their equipment not go into water is out of question.