So there you are, standing right before the Big Bad, but the bullets from your BFG just bounce off his armored hide? No sweat! Or rather, please DO sweat, because any sufficiently large body of water will give you the opportunity to finish him off. How? Just throw him into the water! As soon as he's wet, the battle will be instantly over, and you'll have won. At least if you're in any production rated PG.
Sometimes, you don't even need the body of water; a waterhose will suffice if you manage to soak your opponent from head to toe. He'll instantly lose all fighting spirit and peacefully surrender to any higher authority that conveniently comes rushing into the room right after you've made your enemy wet. Apparently, it's not the act of having to swim (or the fact that said opponent sometimes cannot swim and has to be rescued) that causes defeat, it's the state of being wet itself.
This trope usually works better if applied by a child or teenage hero on an adult - the more respectable the adult before the soaking, the greater the effect. However, if the target looks appropriately silly after getting wet, it will work on man and monster alike, sometimes even on amphibian or piscine monsters who would normally have no problem with water. Thrown them into it or apply a fire hose, and you've got them.
Contrast Kill It with Water, where the target is naturally weak to H2O.
- Whenever there was a river, a lake or the sea anywhere in The A-Team, you could be sure that after ten gazillion shots of ammo fired into the blue (or into cover), someone would eventually punch, throw or kick the bad guys into the water, instantly taking them out of the fight.
- Falling into a large body of water is almost always a KO in Kamen Rider. Basically, it's become a Running Gag for the first enemy who's badder than a normal Monster of the Week to beat the crap out of the Rider and then send him flying into the drink. It's not the water that keeps them down, it's the Curb-Stomp Battle that ended with the dip. That said, Riders who suffer from this usually later emerge from the water in a lot of pain but not seriously injured. This is of course averted for the rare few Riders who have powers or alternate forms that let them operate underwater.
- Avatar: The Last Airbender had this all of the time: Throw a bad guy into the water, and he won't bother you any more. Justified to some extent as the bad guys were usually from the Fire Nation...
- Truth in Television. As getting back out of the water would take some time and the person on dry land is in a very good position to prevent you from getting out, falling into sufficiently deep water can actually cause defeat, at least in a one-on-one situation. Not because you are injured or your morale is broken, but for the very practical reason that continuing to fight may very well allow your opponent to drown you.