Disclaimer: Naruto isn't mine.
It was a marriage of convenience - on more than one level.
Haku had just graduated from medical school and was trying to jump-start his own profession. Zabuza knew from personal experience that such a thing was next to impossible; even after eleven years of working as a chiropractor, business wasn't always steady.
And while the kid was the most talented acupuncturist that he had ever met, most people wanted someone older-looking as a doctor. Most people, no doubt, suffered under the misguided assumption that older-looking meant more experienced and, as a result, more skilled.
But Zabuza had seen what Haku's hands could do with a set of needles, and he had complete faith that the kid would make it big in record time. Until then, however, Zabuza hoped that they could continue to exist as did, with nary a soul to bother the two of them.
Except, perhaps, the occasional client.