Hi,
I don't know much about bottom conditions in your area, but I can get profiles in glacial clay down to 10 meters using the 50 kHz channel. The transducer can't paint a detailed profile because of the large width of the beam. For better detail a large transducer element, or a Mills cross is needed. Then it should be possible to do sub-bottom profiling in not too hard seabeds.
Rickard