The problem with trying to pop a new through- the- hull transducer in is just that - not only will you have a fire hydrant gusher of sea water on your hands, which makes it tough to push against that water pressure, it's my understanding that the outside flange of the transducer is bigger than the diameter of the hole. Otherwise how could you make it watertight? How can U push the sensor head through a hole, if its too small? It would have to mount through the hole, from the outside of the hull! You're better than me if you can do it in the water. That's why I went to the Garmin unit, as it mounts inside the hull. The problem with those is that you must have a perfect connection to the hull - no air voids or it won't work. I wound up reusing an older mineral bath filled dome that one of the old, abandoned transducer used. I just cut a hole in the top of it with a dremel tool, popped it in, and then sealed with silicicone caulk.
I tested the location first by setting the transducer on a sandwich bag full of mineral oil in the bow, beneath the water tank, which confirms that there's no air void where U wanna install it - this worked 4 me. :naughty:
Good luck - J