For average regular target or commercially manufactured defense or 'military' cased ammunition (shotgun shells excluded) there is no reasonable and physical round construction reason to expect that humidity, a drop of perspiration (!), or even being splashed never mind submerged in water would render the ammo to be non-functional.
It takes a long time for brass and steel to corrode through and through, even when exposed to sea water.
And aluminum generally does not corrode at all outside of being exposed to highly acidic (higher than human sweat) concentrations. This latter item is well known and documented in the auto industry with aluminum alloy rims and drivetrain components; including _aluminum_ radiators, blocks, and heads that are permanently exposed to water concentrations. They generally do not corrode through and through.
BTW brass does oxidize over time and when exposed to water, but that is not to be mistaken as being corrosion.
A wholly different matter and functional concern.
Any former Coast Guard, Navy or Marine as well as a commercial sea man can comment same.
I see this topic comes up a lot on gun forums as a what would happen if theory item.
But never have I seen anyone actually report real world an actual problem for same.
Heck I think what I may do myself is stow two rounds each of my own ammo; .22, 9MM, 45, and .223, all being commercially manufactured in a jar of 1) water and 2) brine salt water for a time period (say one week) and see/show what the real world functional results are.
I'm all in on both will fire as expected and at if not near normal as dry FPS, without an issue.