Most of you guys use your boats in freshwater, so, sealed lights might actually hold up.
But I've adopted the saltwater boater mentality, in this environment, NOTHING will hold up. Been running in salt water for about 30 years now, and I've seen this stuff eat everything, even stainless steel under the right conditions. I have a 1.5 inch stainless propeller shaft from a commercial fishing boat, that suffered from intergranular corrosion and electrolysis, it weakened to the point that it snapped cleanly in two! So, if it can do that to a massive piece of solid stainless shaft, you know what it will do to smaller components.
Even the best quality trailer lights will not last when used in saltwater, even if you are washing the trailer every time you use it. The problem is that while you may wash the trailer when you get home with it, while you're out on the water, that salt water creeps into every crevice of the trailer, and the sun bakes it, as the water evaporates, what's left becomes even more corrosive. If a seal in a trailer light deteriorates the slightest bit, and salt water enters that light, that's the end of it, it may not happen immediately, but it will happen soon enough, even if you wash the light out with fresh water, even if you have dielectric grease, etc. There is NOTHING you can do to protect light assemblies from salt water, except to put them up on guides.
