If you have two people, each person should simultaneously take a dog by the scruff of the neck or collar with one hand, and with the other, pull at the dog's flank, where the rear leg joins the body. The dogs will let go of one another and turn towards their flank, thinking another dog has attacked them. When they do this, each person should swing the dogs around so they are no longer facing one another and walk them to separate enclosures. Don't just let them go or they will go right back to fighting.
If you are alone, slip a leash under one dog's stomach and run the end through the handle, creating a noose. Tighten it up and then back up, and attach the other end of the leash to something solid ... a post, tree, fence, etc. Then go and take the hind legs of the other dog and steadily pull backwards until the two dogs are separated. Take the dog you pulled off and isolate him in a crate, car, or other enclosure so that he cannot return to the fight.
Water usually does not work unless the dogs are not particularly committed to the fight, are very small, or the stream of water is very strong. (Think fire hose.) Stun guns don't work either, they just make them fight harder.
Unfortunately I have "been there, done that" a few times too many ... this is the only way I know of that works!