Waterblade

Nya Murchadh was raised in an orphanage for boys, which suited her just fine. She didn't like playing with dolls, she could hold her own in a fight, and she loved to invent.
Her life is difficult as she navigates the ups and downs of being the only girl.
One day, after escaping, Nya meets an old sensei who offers a chance at a new life.
Now Nya goes from a neglected girl in an orphanage for boys, to a water ninja in a monastery with caring, chatioc boys.
The story is better than the summary. I think.