Waterproof, but not corrosion proof. I keep a Legend Cx on a Garmin suction cup mount on the deck. The suction cup is pretty tenacious on gel coat.
The GPS battery compartment is not waterproof. Salt water will corrode the terminals as well as get the buttons crusty. My first legend failed after 3 yrs. Don't know if water had anything to do with it. By that time, the rubber casing was coming loose as it does eventially on all eTrex GPS. I planned to wrap a sandwich bag around my 2nd Legend, but I always forget.
I wouldn't worry at all about fresh water, but anything that is in salt water...gps, radio, pfd, boat...should get a thorough fresh water rinse as soon as possible. If you're obsessive, take the battery cover off the gps and dry out the compartment.