Whoever said Zabuza was weak? I don't really recall anyone saying that. He's not weak, he's a master assassin. I think he'd easily slaughter Kisame, he'd use stealth and assassination techniques and take Kisame's head off before he knew what was happening. From what we've seen Kisame only uses brute force and charges into battles - no match for Zabuza's style.

And you've got to remember - the reason Sasuke/Naruto/Neji seem stronger than chuunin level nins is because of poetic license (is that the correct term?) - they're the main characters, of course they're going to be stronger than no-names.