adding water will make it taste watered down. you should add water before fermentation is heavily underway, so it can be part of the chemical conversion process.
at this point, you'd be better off brewing a couple gallons of the same brew, with a lower gravity, and mixing the two.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10