Only 3/8 hole?...You're a lightweight!
The key element is that your patch include mechanical clamping force and some sealing area like a stainless steel fender washer and then seal with 5200which is removable later.
Thank you for introducing this issue as it presents an opportunity to relate my daring adventure wherein I personally drill a FREAKIN 2 INCH hole in my hull while in the water, INTENTIONALLY. Apparently this is a common way to install transducer thru hulls where haul outs are costly and maintenance divers are available. But, I had no diver.
Bronze thru hull and nut, teak washer for inside hull, 5200 sealant, dummy for transducer, wood safety plug sized for the hole, 1 1/2 inch fishing bobber, 15 feet of braided fishing line, a 3" diameter, 1/4" thick plywood washer drilled to accept the fishing line. dingy, boat hook, drill motor, hole saw, spanner wrench. Labatt's Blue.
One dingy man stationed in the dingy floating abeam of the planned hole, One GFI man to unplug the 110v drill if I start shaking. One Travel Lift operator on call in the event of an emergency. (caution, travel lift operator may be a little cranky 'cause you're stealing his opportunity for a haulout so know your man).
Announce your plan at the prior evening's drink and bull session on the dock. The next day, you can expect a large audience hoping to see you sink your boat. However, discourage photography as the graphic evidence in court woud be too damning.
Caution: Distract the admiral by sponsoring a cloths shopping foray into town.
Caution, Keep Labatt's sealed until the job is done.
Dingy man is on station with the tranducer thru hull. The flange is slathered up with 5200 (not the threads). Boat hook at the ready. GFI man is stationed over my shoulder drill motor plug in hand. I drill the hole. Upon breakthrough unplug the drill. The gyser is only about 4" and I easly stop flow with the palm of my hand. I push the bobber into the hole trailing the fishing line and and close the hole with my palm. It must float up the under hull a bit. Dingy man snags and pulls the line from the water, disconnects the bobber, threads the line through the bronze thruhull and then the 3" plywood washer and secures with a proper knot. Dingy man then holds the thru hull at the waterline while I take up slack and a deep breath. On my order he drops it. As it sinks I quickly slip the line in through fingers of my palm. Once the transducer is up to the hole I shove two fingers into the ID to square and bring it through. Next, Holding the transducer firmly to the hull, I insert the dummy plug. I start the teak washer which has a bead of 5200 and bronze nut. I draw it it up with the spanner. Exhale and open the Labatt's
Over the next few weeks and months the story is retold and embellished by the witnesses over evening drinks. I sit quietly with an even stare into the setting sun as if in deep thought , deflecting accolades with the simple wave of one hand. Eventually, I am annointed ........Count de Monet!
"I don't know where I am but I know I'm not lost"