FWIW, early models like the 601, 602 and early 603 had non-drain hole screws - something guys doing retro builds actually look for. With input from SEAL teams and others, the M16A1 was standardized with a buffer tube screw with a drain hole, which doesn't do anything for clearing the barrel, but definitely helps drain any water in the receiver extension. Along with chrome chambers and better extractor springs, probably one of the more useful improvements made to the weapon.
As an aside, the Mk4 Mod 0 had a Kal-Guard finish on all surfaces to inhibit corrosion, and was rated to I believe, 70 meters unprotected. FWIW, Victor at US anodizing offers a 'Navy finish' that is likely as close as you can get - although any Moly finish is pretty mmuch the same thing, apparently he does EVERYTHING, including BCG and TG, which brings up interesting tolerance issues.
I doubt a SEAL would ever put a condom on weapon for such use, since as many have said, you want to charge the weapon slowly to let it drain - a condom would be a problem there. That is an old school way to keep out grit however. I can see them using bags, but always used to laugh when I saw those Ranger commercials back in the '80s where they rise slowly out of the swamp, weapons at the ready. Definitely drain the barrel. Not as glamorous as bagging the weapon though, LOL. ;)

