Hazardous Water is a horror trope. It means that a character in a dangerous situation in a horror plot who falls into a body of water will have great difficulties getting out again before the monster lurking in the water attacks him.
Also, water plays absolute havoc with the ballistics of pretty much all ranged weapons - especially firearms - so fighting anything in the water is liable to be tricky.
The water itself may also be literally hazardous - typically through being funky with something toxic or infectious. For example, where the water is, essentially, an open sewer (as in many urban areas in many eras) falling in the water can lead to exposure to all manner of nasty diseases - and even "natural, clean water" can have all sorts of things in it that you don't want getting into an open wound. Or your mouth.
Game and Story Use
- Sometimes, it's best to ignore distance and movement rules for monsters, and just have a monster attack when a character falls into the water. Anything to increase the tension.
- Besides, if the water is opaque (like in a swamp), then it's not as if the characters can prove that there wasn't a monster just moments before…
- Water is generally a good place to store monsters, it's just a question of those that will (at least partially) emerge from the water after passers-by and the opportunistic ones that will quite happily stay in the water for ever minding their own business until something lands on them.
- Can be played remarkably straight in most fRPGs given that PCs tend to insist on wearing the heaviest possible armour at all times. Even without armour, most natural bodies of water have a thick, sticky layer of mud and/or organic silt at the bottom which can make them surprisingly hard to get out of.
- Great for subversion for SBS/Navy Seal type characters who can be the monster in the water.