It seems the intention was to create a father-son relationship, but roughly 400 episodes later, I am less convinced that it was. Haku wanted to protect someone, and by chance, that person was Zabuza. He went so far as to sacrifice himself in order to protect Zabuza. And on the other hand, Zabuza attempted to eradicate his feelings, but when Haku died, he went into a blind rage, killing dozens of people.
I suppose a comparison would be Gai and Lee. Lee admires Gai and is thrilled to train in order to make Gai proud, and Gai is thrilled to have such a faithful apprentice. But the connection is not the same as Zabuza's and Haku's connection. There is simply... something else there.
Am I looking too much into it, or does anyone else think there may have been romantic feelings, at least at the end?
I think it is up to speculation and depends on how each person sees it. Seems much more than just a father-son like relationship to me. Haku seemed to have loved Zabuza, enough to give up his own life to save him and would do anything for him. That type of love would have to be strong.
Also, at the end, Zabuza showed emotions that he hadn't showed until Haku's death, which says that deep down he really did care for Haku, even going as far as killing Gato and his men even when injured and said that he wished to go to the same place as Haku.
The whole scene wasn't just sad, but full of emotion and very touching. It is said that love gives people strength and courage and Haku said something similar to Naruto about that and finding someone to protect.
Unlike Gai and Lee, their feelings aren't hidden and is more of Lee admiring Gai/Guy, wanting to make him proud and Gai seeing Lee like a son it seems. They even look alike, possibly to imply their father-son like relationship or Lee's admiration.
The way I saw it Haku loved Zabuza but Zabuza only saw Haku as a weapon and tool. Yes he ended up not being as heartless as he thought he was but even then he probably saw Haku more like a pet then anything else.
Zabuza only though of Haku as a tool. But I think Haku was in love with Zabuza. He was dressed like a woman anyways. But I don't think it was intended for the audience to think that they were in love or anything like that.
Yes, you could say its romance, but Naruto is a Good anime, that i could never see on purpose putting a gay couple on, it was more of a response of this is my fault! I meant so much to him, and i threw him away like nothing.... I dont see it as a romance, although it would've been cooler if haku was a girl i think lol xP