Sealant choices

I'm sure this has been answered, but I'm having a hard time filtering through a lot of the junk/poor advice.

I'm in the process of replacing my thru-hull transducer (I've upgraded from an old depth sounder to a nice garmin chartplotter) and I'm trying to decide what to seal it in place with. I'm working with a solid fiberglass hull and a brass transducer (Same diameter of the old one thankfully).


I'm not trying to debate using silicone below the water table, I'm just siding against that.

I'm also not using 3m 5200, so don't bother posting that. Being that I'm restoring my boat and I can understand and appreciate the need for everything to come apart one day.


So what does that leave me with?