Objectives with high power magnification have short focal lengths, facilitating the use of water.
Electrophoretic preparations used in the case of comet assay can benefit from the use of water objectives.
The refractive index of the water (1.33) is closer to those of imaged materials or to the glass of the cover-slip, so more light will be collected/focused by this type of objective comparing to air-immersion ones, leading to a range of higher numerical apertures (NA).
[1] Unlike oil, water does not have the same or near identical refractive value as the cover slip glass, so a correction collar is needed to be able to variate for its thickness.
Lenses without a correction collar generally are made for the use of a 0.17 mm cover slip or for use without a coverslip (dipping lens).