I actually read an amazing theory about this that convinced me of your point. Look at Haku's weight it's roughly the same as the k11 girls in shippuden, the deception thing is correct (if she tells people she battles that she's a boy the hunter nin would be looking for and questioning people about a man traveling with a boy), in some panels haku has small breasts( its not just the short it wrinkles like small boobs), haku once talked about zabuza's beautiful body, zabuza and haku clearly loved one another, zabuza seems like the type to train a kunochi but not treat her like a girl in an attempt to make her stronger (most kunochis tend to be weaker), as he said haku emulated her mother which wouldn't make sense if he was a boy, zabuza ran game on her (Sakura told haku how zabuza acted like when she died and haku cried and zabuza got mad, he wanted to play like he didn't want her) and zabuza was very ambitious it makes sense he would want a powerful woman that could give his children the ice kkg.