You should not need to worry about the frequency. It will impact how deep you can get readings and the width of the area sampled, your system should adapt to the signal.
All you really need is soundings down to 50 feet.
This might help to understand the difference in signals.
Transducer Frequency
Transducers for recreational and light commercial boats usually operate on frequencies between about 25 and 400 KHz, with 50 and 200 KHz being the two most common. As with most radio or radar transmissions, lower frequencies generally have greater penetrating power while higher frequencies offer higher resolution or detail. A low frequency around 50 KHz is excellent for displaying a wide area of the bottom, especially if the water is deep, while a higher frequency shows more bottom detail in shallow water, a very useful trait for fishfinders and recorders. A depth sounder signal that reads up to 400' in fresh water may lose as much as half of its penetrating power in saltwater, so lower frequencies may be favored for saltwater use.