As Josh stated only distilled water should be used for topping off the battery, then charge the battery for an hour or two, at 5 amps to mix the electrolyte.
But you should not add water if the plates have dried out, from being low on electrolyte for a long time. This will just water down the electrolyte and the the battery will probably not generate enough current to crank the engine in a day or two. I don't check the specific gravity any more, as the source voltage testing gives a good test of the battery condition.