I've always maintained that metal detectors should be waterproof. Why? As Dan says above, our machines get dirty and its always nice to be able to spray them off without worry. Secondly, since our hobby is an outdoor sport, rain is always a potential threat to the internal electronics. So even if you don't physically hunt in the water, getting caught in a sudden Florida afternoon rain is a problem. It sure would be nice to remove that concern regardless of where you hunt.
My first Minelab was the Safari. A nice machine but not waterproof. So even when I searched ONLY the dry and wet sand and had a plastic bag covering the control box, I was always paranoid about salt spray, dropping it or some rouge wave taking me out of the game. When I got the Excal and CTX (with the new gasket), my worries were gone and I enjoyed the hunt that much more....and...I could hose them off or take them into the shower for clean up without concern as Dan B says.
Just the view from my foxhole...