If we connect a source of electricity in a large water body ,will it be dangerous? I was wondering whether a person will be electrocuted if he is in large pool or a sea which is connected through a source of electricity . As sea water is saline it will conduct electricity so will it be dangerous !? 
 A: Yes, especially in fresh water bodies.
Very tragic: http://abcnews.go.com/US/fourth-child-dies-missouri-tennessee-lake-electrocutions/story?id=16714407
According to Electric Shock Drowning … The Invisible
Killer, the danger is especially in fresh water because the human body conducts electricity better than fresh water.  15 milliAmps is enough to lose muscle control and drown.
A: TO expand on the above answer further (and add a little physics)
If I have a loose wire in the water, some of the current leaks out. That crates an electric field. The electric field can be considered as roughly spherical and decreases with distance. 
When you swim into that field, the salt water in your body is more conductive than the fresh water around you. That means the charge carriers (electrons) just love you. They all want to be your friend. Unfortunately that can kill you. 
The reason is that by being more conductive, you're actually creating a potential difference -- there's now a voltage across your body, which completes a circuit to the ground. 
The current levels can vary, but it only takes a few milliamps to kill. (About ~30 will do). That's 30 thousandths of an amp. A house circuit runs at 15 amps, and the ones in a marina will operate at single to double digit amperage levels.
In salt water the situation is a bit different. The salinity of human bodies is about on par with salt water (actually a bit less) so in that situation you are less conductive than the water. On the other hand water with salts conducts electricity better than fresh so the field you need to worry about will be larger. 
