Fishfinders vs depthsounders
You can't
and
Won't happen.


Sorry Stu, but with a little trial and error using plumbers wax or vaseline, a transom-mount fishfinder transducer can be installed inside the hull forward of the keel. How far forward will depend on the length of the transducer cable and where the display is mounted. Fishfinder transducers are small so finding a good spot in the hull should be easy.
A fishfinder display also shows the bottom profile. In a shoal area, you can see how fast the bottom is coming up, which gives you time to take appropriate action before you run out of depth. Fishfinder scan rate can be adjusted by the user and should be set at the fastest rate in a shoal area.
Finally, most sandbars in Florida have gentle slopes, which provide fair warning before you run out of depth. Of course, in places with rock bottom (Keys, Bahamas, Maine) you gotta pay more attention.