Assuming you need to gather displacement data from every position along the horizontal dimension of the tank simultaneously, you may be looking for a cylindrical lens, which focuses light onto a line, as noted in the link. With a proper lens setup, you should be able to change the aspect ratio of the field of view and capture greater resolution along the vertical without affecting the horizontal.
It might be worth putting a uniform, high-contrast background behind the tank and using plenty of illumination of both the water and background to assist with accurate measurements.
An alternate method to consider is interferometry, and it may offer greater accuracy than imaging devices. From the link (emphasis mine):
Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes and surface irregularities.
I am unsure how to set up an interferometer to measure displacements in water height, however.