If the boat is moving very fast and the rate that the sonar unit is pinging is slow, then the number of times sound is bounced off the seafloor in the area covered is fewer than if the boat was moving slowly while the unit was set to a high ping rate. Also, if the area being covered is very deep, then the boat must move slower over that area, or the unit must use a higher ping rate, in order to achieve an equal quality image as to images at shallow depths because it takes longer for the sound pulse to return to the transducer.
The ping rate is also limited by the speed of sound in water, because with most multibeam systems the transducer must wait to receive the last ping sent before generating the next one (i.e., there can only be one ping in the water at a time). This operational constraint sets an upper limit on the ping rate for a given water depth and swath width combination.
Chuck