Pretty much anything can affect point of impact. Water, grass, seeds, sticks, fouling, dust, oil, bugs, etc. We even have a species of bug out here in Arizona that likes to make its coccoon in small round holes - kaboom!
Water in the barrel of a rifle changes the resistance to the travel of the bullet through the barrel, thus changing the point of impact. Any material in the bore, such as powder residue or fouling, can absorb water thus increasing its size and resistance. Wet barrels can even be dangerous if there is enough water in them.
When hunting, I try to keep water out of my barrel as best I can, usually by dumping it out periodically. A couple of times the bullet in the chamber became discolored, or had a ring on it from the beginings of corrosion from water. This tells me to run a swab through the bore with a slight amount of oil on it. (This changes the point of impact too - some hunters insist on taking guns to the field with dirty bores from the sighting range) I have no idea what effect it actually has on point of impact, since I am not likely to conduct experiments. WWII GIs supposedly used condoms to keep water out of the rifle barrels, but it looks too silly for me, and the condom itself would affect the point of impact.
I have read about some bad accidents involving flooded barrels and "underwater shooting" experiments, but I have never seen one in person. A water-filled barrel is pretty much an obstructed barrel, and common sense says that this is a Bad Thing. I have heard that some guns are designed to fire underwater, but I have yet to see one do it.
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