Lynn, the main character is flawed without being stupid or a martyr, strong without being unbelievable, and naive without being a total bonehead. In other words, she seems like a believable 16 year old raised with only her mom (and occasionally a neighbor) for company in a world where drinkable water is now more valuable than diamonds.
The world building is also very good. Lynn lives on a farm in Ohio that has a small pond, which she must defend at all costs if she wants to live. Others have relocated to large cities, leaving vast swaths of the U.S. uninhabited. It's realistic enough to draw you in and make you feel just how desperate her world is.
I do with it was longer though. I know there's a second book, so I'll have to pick that up next.