I've found that a little vaseline or other waterproof grease does wonders around the battery compartment lids and cable sockets with rubber caps on these GPS units & on my marine radios.
I've sailed for many years now with a Garmin GPS III+ mounted on my boom for hands free monitoring of my course. The mount lets me swivel the unit after a tack. I can read the display (set to large numbers in compass mode) from the rear corner, and even while trapped!

Try it out!

Mike.