Here's what I would do: Replace your thru hull
unit with your ultrasonic unit, then put an inside the hull unit in for depth. This assumes you have a solid glass hull. You can try the unit out by using KY or something similar and placing it on the hull.
An alternative is to drill the thru hull
while in the water. It's scary, but not all THAT bad. This also assumes you have a solid hull with no core
. A cheap
portable drill is all you need (you will likely kill it when you punch thru, and you don't want to use a 110v drill). The heal of your foot over the hole or your hand will seal it once you punch thru. Then have a diver friend push the thru hull with a plug
in it thru the hole, haul it in, and seal it. Use an in-the-water sealant
. Others might chime in here.
Remember, hydrostatic pressure is your friend. If you are 2 feet under water, you have 2 feet of hydrostatic pressure. Imagine a piece of 3" PVC filled with water, and your hand under it. That's all the pressure/volume you are getting.