Re: Submersible lights?
Yes, if they were not waterproof they would burst as soon as the water hit them. Or they would short out.
Newer waterproof lights have an airtight chamber (depending on brand) which separate the water from the light bulb. These seem to work quite well, based on the ones on my trailer, 3 yrs and counting.
Older ones (again brand specific) used the "air chamber" effect to keep water out. The light housing was open to the water at the bottom but sealed on the edges and top. This formed an air bubble, keeping water out by positive pressure.
These didn't work so well, again based on personal experience.
I haven't used LEDs but based on my reading of "posts" and friend's experiences, they are very sensitive to ground contact and current flow. The only thing I dis-like about them is when they start losing a "lite" they look bad. Personal taste.
In any case, the connections, ground contact, and quality of installation will determine the life of your system. The best setups are the ones that run a separate ground wire to each light, and not use the trailer for ground. And make those connections waterproof.