While I am sure that all of the above answers are technically correct, I would trust only the ones that report actual experience.
My experience is that it is usually hard to tell that anything is happening with short (seconds to minutes) exposure to fresh water, brackish water or sweat. I am sure that some current is being drawn which will reduce battery life, but no bubbles of gas, no heat, no swelling of battery.
Can't really report any personal experience with exposure in the minutes to hours duration range.
Longer exposures (days to weeks) do result in corrosion of exposed metal parts, but no noticeable heating or swelling of the battery. Battery life is definitely impacted, and often ended.
Very long exposures (months to years) turns the battery into a nasty corroded pile of gunk.
All of these observations are subject to change under conditions other than what I have observed. For example the nasty corroded pile of gunk stage can probably be achieved in only a few days or weeks under some conditions, say a hot ocean beach for example.