What is the question here..........if you are asking what the galvanic corrosion issues of different metals are in seawater it is more for metallurgists not chemists as far as I am aware. Good article here;

membership.corrosion.com.au/wp-content/uploads/2017/08/CM-April-2013-Galvanic-Corrosion-Dissimilar-Metals-Technical-Intro.pdf

Am sure you are aware of all this. Another useful article here;

www.industrialgaskets.com.au/flange%20and%20bolt%20insulation/page/shop/browse

We are all aware of the Duralac jointing compound for building masts but there are later alternatives........

www.tefgel.com.au/

So I have a very poor quality aluminium alloy in the bracket near stainless alloy immersed in salt water
but that alone would not give me the crystallisation in 48 hours unless there was charge in the water
maybe ground leakage from the 240 on the pontoon
just seeing what the brains trust thought

A lot of good articles on line.
www.boatus.com/expert-advice/expert-advice-archive/2019/october/galvanic-isolators-explained yachtsurvey.com/corrosion_in_marinas.htm
Depending on the brand those transom transducers can shoot through single skin glass hulls hence avoid the issue of the transom alum frame and ss fasteners. See plenty of US websites showing the best methods. See here link to shooting through wooden hulls
forum.woodenboat.com/showthread.php?107450-Using-depth-sounder-through-wood-hull