Zabuza would most likely win since this location gives him the advantage. Katsuyu won't swim in the water (or drink it) as this will cause your blood to become diluted, thus the level of salt concentration in your blood drops. Salt concentration in your blood becomes lower than that in your cells. This is slightly drinking more then one can. Now imagine a creature made of 90% water drinking an entire lake, I don't know how to put this, for one it is not possible, and second of all it would die, conclluding to Zabuza cutting her open and releasing the water contained in her.
Sakura's punches are borderline effective in the water. Her punches will also be desperate and aimless punches due to her impaired vision from Kirigakure no Jutsu, that even hinders a sharingan. Furthermore, Zabuza is more intelligent then Sakura, giving him an even greater advantage. He demonstrated his high intelligence when fighting Kakashi during part 1, coming to a point where he was capable of trapping him in the water prison. He has more will, higher intelligence, a better arsenal, and a location advantage. All Sakura has is brute force, decent intelligence, and healing techniques that won't suffice her.