well, certain pistols can be fired (and will cycle) underwater, like a glock, but only if its got the maritime spring cups, and ONLY with JHP/ball ammo. And you'd better have your head ABOVE the water when you fire, or you'll likely be deaf the rest of your life, not to mention possibly die from ruptured eardrums.
As mentioned above, to summarize:
1. fire an AR underwater, with condom on then end of barrel, first shot ==> will fire, will not blow up, will probably not cycle
2. fire an AR underwater, with condom on the end of the barrel, subsequent shots (since condom is now broken and water is in the barrel) ==> bore obstruction; will fire, but WILL ALSO blow up; cycling becomes moot at that point.
3. fire underwater, no condom, same result as 2 (obviously).
4. dunk then fire, with condom on end ==> will fire repeatedly, no problems, mate.
5. dunk then fire, without condom, without doing the water-clearing procedure described by 44 amp (tilt forward and break seal with partial pull) ==> bore obstruction; will fire; but WILL ALSO blow up; cycling moot point.
6. dunk then fire, without condom, AFTER doing the water clearing drill ==> will fire repeatedly, no problems, mate.
So generally, no, this is to be avoided. Water bad. Well, as mentioned, a little water/moisture in the barrel is no problem. But enough to obstruct the bore completely is where you have problems. And as with pistols, if you're head is under there, that is very bad mojo.
I refuse to address the outcomes if dunking other items besides the rifle, after placing condom on other items besides the rifle. ok, dumb attempt at humor....