I thought I'd relate what I discovered last weekend. I installed an outlet on my deck a few years ago. I mounted the "waterproof" box under the railing and ran the circuit to a GFI breaker. All was well until a week ago I realized the circuit was out. I checked the breaker and it was tripped and whenever I reset it it tripped again. I thought great the breaker went bad, but checked the outside box before I did anything else.
Much to my surprise when I unscrewed the cover, about 3/4" of water ran out. The water was up to the contacts of the outlet, causing the GFI to trip. I cleaned & dried everything, drilled a few holes in the bottom of the box and put it back together and everything works fine. I have no idea how the water got in, but at least it can get out now. When I get a chance, I'll look for a better box to replace the old one. I'm glad I had it wired to a GFI breaker.