GPSMap 76cx leaks

It seems to me as if a IPX7 waterproof (not to mention floating) GPS would not leak from getting splashed - yet I'm on my second unit (replaced under warranty) and it just failed to turn on because the batteries rusted overnight - there were (salt) water droplets in the battery compartment. It worked fine until I turned it off the night before.

My unit is used on top of a Prindle (think Hobie Cat) trampoline. It's strapped down, and gets splashed by waves. After use I wipe it down and toss it into a dry bag. Next day I might discover that it has water in it, however little. This time I wiped the battery compartment and stuck it in a dehydrator at 115F overnight, and it works fine, but it's still not nearly able to live up to its claims of being a trouble-free waterproof unit. (And how many people have a dehydrator?)

The unit has only been used twice since I got the refurbished replacement, and the original new unit also didn't last long, with the same exact symptoms. I never open any seals near water, nor do I even open them at all - I put in batteries at home, and it was still on the first set. There was some water under all 3 rubber covers - battery, USB, antenna. USB jack is now white (I assume that it's aluminum).

One thought that crossed my mind was that the unit sucks in water with cold/hot cycling (vacuum created when it cools).

Anyone have similar issues? Suggestions? Did I buy the wrong device for the task?