Salt water will no doubt make a GPS not water proof after a few dunkings. The salt makes the rubber stiff and crack even after only a few uses. TO keep this from happening, you can spray the outside (not the screen) with a dry silicon spray, or you can wash it when you get home. I have had similar problems when i was doing some mapping for my university out in seattle. Washing it daily is what Garmin recomends. For the battery problem, i purchased some good energizer rechargable batteries. I would use my unit for 10 to 12 hours at a time and it would last the whole time. It only takes an hour or two to recharge and the charger automatically shuts off when charging is done. Presto chango.

Reid Rechel
N5.2
Lake Superior