I think this might work extremely well at low speeds.
At high speeds, I tend to agree with Rickie:
If you have it too low, I think the bracket
could get bent, or even shear completely off
from the force of the high speed water.
Regardless of speed, if you have it too low and
hit something, or run a ground, I think you will
be without a working transducer the rest of
the day.
Depending on how you mount the transducer
to the bracket itself, you could get some
unwanted water turbulence around the
transducer.