Ok I see what you have and trying to do. Just to recap what I see in the photo, you have a surface mount transducer with the instrument wire that is routed on the outside of the transom of the I/O boat
. This wire is currently routed through a hole close to the waterline. Is that correct?
Couple things come to mind. If water
egresses into the hole, it will surely compromise the 1/2" wood core
and the transom integrity will suffer or fail completely. As you know this is not good.
I'm going to refer you to a utube channel "boatworks today" within the last couple months of weekly vlogs, he has completely disassembled and rebuilt a Burtrum runabout transom that had water intrusion into the core. Good source of real life issues of boats.
Anyway I tend to agree with some of your previous advice suggestions. It's really hard to seal a wire by itself going through a bulkhead, transom, or deck
especially if it potentially can be submerged. There are special through deck
fixtures that may help.
I've also seen round rubber seals
of similar construction for wire penetrations. Don't believe either is rated for submersion.
Usally for something like this application you see the instrument wire enter up higher on the transom or even on the deck where the possibility of submersion is eliminated and only water splash or rain is the threat.
So permanent fix, in my opinion, would be to pull back the wire through the current
hole. Prepare that hole for fiberglass, core, fiberglass repair. Coat with color match gel coat, buff and drill new hole higher up on transom using a seal similar to above. Google
Gougeon West System. On their website you can download several manuals
on how to prep and repair fiberglass holes. Quite detailed. They also have a free knowledgble technical staff you can call.