My gut feeling is that it's not necessarily strong jets of water that causes the problem (although that would probably do it too), but just from being damp for days at a time. From waves washing over the deck, to just paddle drips, a GPS can stay constantly wet and water eventually finds a way inside. In many ways just being wet for days is a much tougher test than just a one-time 30 minute immersion.
For example, I had a Brunton Eterna "waterproof" monocular that worked fine for many day trips, including immersion, but it literally fell apart after just one week in a slightly damp under-deck bag, when I was going around Newfoundland.
For daytrips in good weather I often don't bag my GPS, but I do for any extended trip. Visibility is impaired but having a GPS mount to change the screen angle makes it easier to read (my mount is made from scrap foam, bailing wire and duct tape and weighs almost nothing). I use the CVS lens wipes that I bring along for my sunglasses to remove the salt and oil smudges from the bag window. I throw one or two silica-gels in the bag to absorb water vapor. So far I haven't fried any of the four Garmins that I have owned, and am currently using a GPSMap78sc, with a GPSMap78 as a backup.
That said, sometimes I simply can't read the GPS while in a bag (Aquapac), and must remove it. That happened to me last year in Whitewater Bay due to a combination of bright sun, salt streaks and glare.
Also, it probably should be said that modern electronics sometime fail for no apparent reason. My brand new Olympus TG2 camera failed recently during a backpacking trip in very dry conditions. One minute it was working, and the next it was fried. Had the same failure occurred in my kayak, I would have assumed immersion.
Greg Stamer
