I would heartily recommend a Tack Tic and get the transducer that shoots though the hull. Then epoxy the transducer to the hull.  Sound travels much better through a sold then the semi-solid caulks.

And there are two advantages to the Tack Tic. First you have fewer wires to run and second when anchoring you can walk with the unit to the foredeck and see what the depth is immediately as opposed to always yelling back to the helmsmen. :)