Not bad at all. Gratz on your first AG.
You can definitely add water, but I don't know how low the gravity will fall with just a half-gallon. I know there's a formula/calculator for figuring that out, but I can't find it right now. I'm thinking that will drop your gravity by around 0.003. Maybe someone else on here knows the formula.
I'm also assuming you haven't fermented the beer yet. Adding water afterwards can be a little tricky because of oxidation.