Bong water has a very distinct smell, though it can vary slightly according to the type of pot that's been smoked through it. It's similar to the smell of pot, but darker and thicker, and of course wetter — like halfway between the smells of stale pot and wet leaves. It's a lingering smell, too; carrying a wet bong (i.e. a bong with water in it) around under your jacket for more than just a few minutes will leave you with a faint but undeniable aura of bong water.
Bong water often has a bits of ashes floating around in it. In some smoking circles, it's common for the bowl to be deemed done by "pulling it through" — that is, sucking all the burnt material though the bowl and into the water.
Bong water also has a slight tinge of green to it.
Because of the three facts above, it's a bit of a pain to pour out a bong when indoors or in a car. When out-of-doors, the ground is usually quite receptive, but when inside, one must usually locate a bathroom (or go outside). The distinct smell lingers around wherever the bong's been empied for a little while, but fortunately the green tint is usually muted enough not to stick around anywhere.
Because of the observation above, many people do not pour out a bong directly after using it, especially if it's a designated indoors-only bong. Bongs often sit around for an hour or so or until someone whose mind has cleared somewhat remembers it, and empties it. If the bong is used more than once a day, it might stay wet throughout the day, until it's emptied after the last bowl of the night. If potsmoking is an all-day-every-day sort of occurrence at your house, your bong could have a resident stash of water that lasts for a few days at a time. (By this time, however, the smell's probably gotten pretty heroic, and even the most dull-nosed indivdual would want that bong emptied and cleaned.)
Because of the behavior above, spilling bong water on anything or anyone is not only a faux pas, it's downright mean. Bong water that's been used several times and has fermented for hours is quite rank, and anything it comes in contact with — especially porous surfaces — will reek for days. A spill in someone's lap means that person's crotch is going to smell all day long. A spill in your carpet will make the room smell, and might even leave a faint green patch. Your homework, a backpack, a towel, the hair of a friend — all potential victims of the stench of bong.
Spilling the bong can be stupidly easy.
Placing the bong in a bad place is the main cause. Under the kitchen table, on an armchair, on the floor in the hallway — none of these is a good place for a wet bong. To keep your bong out of harm's way, look for somewhere that doesn't see much movement, like under the side table at the end of the couch, or on the floor in the corner of the room. (Remember, if something's already on the ground, it can't fall to the ground.)
The other cause of bong water spillage is, of course, people. Don't let your blazed friends hand the bong back and forth until it's been emptied. There's also a chance of breaking the bong, so you might consider just putting it away after it's dry.
Filling a bong with beer ("bongbeer," but not a beer bong) is never a good idea. If you're strapped for liquids, just go without — using a bong as a steamroller (i.e. without any water) is much less sacreligous than using beer. Bongbeer is very bubbly, leaves a nasty residue, and beer already tastes/smells worse than water.