Saltwater eats the Mercury trim guage sending units at a rapid pace here in FL, even after being treated with Corrosion X, saltway rinse, and WD regularly. The last time I replaced mine, one did not immediately work because of a lose ground. After redoing the ground connection it worked fine.
-38 Fountain Trip 300's....
-(SOLD) Scarab Sport Twin Merc 250
-A Couple PWC's
-A Bunch of Motorcycles