First of all, many depth transducers are not removable like a knot meter transducer where you have a dummy plug to to insert after you remove the inner core transducer. Most of them fit into the hull from the outside and are one unit with an epoxy core where the wire exits. So, if you have one of these types, the above source is not applicable. If you cut the hole so there is a tight fit for the transducer or through hull and you used a polysulfide caulk such as Boat Life's Life Calk, both between the mushroom flange beneath the hull and beneath the nut that tightens down from above (caulking liberally), then you should not have a problem. This also assumes you installed it where there is not a curve of the hull (you want a flat surface on the inside. If so, then you make up a wooden fairing block). You can loose sleep if your hole is too big so that the thru hull fits sloppily. Polysulfide caulk cures faster underwater too (with the absense of oxygen).
