Complex sentence: The interference pattern between the two beams when the light is recombined at the observer depends upon the transit times over the two paths, and can be used to calculate the speed of light as a function of the speed of the water.
Split into simpler sentences: The interference pattern between the two beams when the light is recombined at the observer depends upon the transit times over the two paths. Thus, the speed of light in the water can be found as a function of the speed of the water.