As companies cut costs to keep fishfinders competitive, cheaper models have stopped providing cable end covers (like the one below).
Aside from the generous use of dielectric grease to protect from corrosion, the cable connections should be covered when not in use to protect from dirt and particulate.
If your connectors lack covers, consider buying vacuum caps to protect them. I got mine on Amazon.
bellcon posted this remedy on another website, and It has worked well. For additional protection, one can fit a rod butt cap over the whole plug.