WhiteSoxRunner--
People do sometimes add salts to their water/sports drink, but it's not to retain water. You may have noticed that after you've been sweating heavily, there are white marks on your face or clothes. Even if you haven't seen that, it's salt (sodium and potassium and some calcium) that is lost with your sweat, which is why sweat tastes salty. Adding salt (aka electrolytes) just replaces what you've lost through sweating. This is important because if you are exercising for a long time in hot weather, it is possible to lose so much salt that you can feel nauseous, dizzy, crampy or even have heart problems because salt is important in a lot of bodily functions.
That said, you probably don't need to add extra salt unless you are cramping or are having other symptoms (and you are sure that you are drinking enough water), running for a very long time (several hours or more), or exercising in high temps. Most people's diets have plenty of salt for everyday running. And most sports drinks such as Gatorade have sodium and potassium already in them.
If you do decide to try adding some salt into your drink, start with just a sprinkle-- no need to add so much that it tastes salty, because that's just plain gross =) If you decide to start doing ultra-distance running, there are other options to add more salt, such as salt tabs (aka salt pills) made by several companies.
Oh, and no need to "salt load" before running, if you need it, you'll need it during/after running, not before =P
That was a long post =) Hope that helps!
