Final answer:
The wavelength of the sound waves used has the greatest effect on the amount of gain required to obtain a fathometer reading.
Step-by-step explanation:
The factor that has the greatest effect on the amount of gain required to obtain a fathometer reading is the wavelength (λ) of the sound waves used. According to the equation f x λ = c, where f is the frequency and c is the speed of sound, if the wavelength decreases (i.e., the frequency increases), the gain required to obtain a fathometer reading will also increase. Similarly, if the wavelength increases (i.e., the frequency decreases), the gain required will decrease.