Thanks for the responses everyone... No extra water it is. All and all a 1/2 gallon lost isn't bad considering my total lack of experience at this point :)
My first batch has just finished fermenting (going to let sit for another week or so) and I lost about a 1/2 a gallon through blow-off. I was wondering if I could use like a half gallon of water for my priming sugar come bottling time to get back closer to the 5 gallon mark? Would there be any...