Wet Lights
I don't think you can make trailer lights water proof. It would be best to keep the lights dry. On a Windmill class sloop (15') we used to mount the lights on a 2x4 that went across the aft end of the boat. It was carpeted so as not to mar the deck. The license plate went on the bar too. It was also used as tie down. The bar had holes in the end where we used a thru eye bolt with short chain and cheap aluminum hardware store turnbuckles to hold the boat down to the trailer. The wire was then lead over the deck or whatever to the tow vehicle plug. We secured the wire by wrapping it around whatever was convenient. You could also use Velcro ties.
Alternatively some power boats use PVC guide-on poles outboard of the hull and have the lights mounted on them well above the waterline and also more visible to following drivers when there's no boat on the trailer. (I've had my trailer hit several times when stopped at a traffic light because without a boat on it, the trailer is too low to be noticed by IOCPs, idiots on cell phones)
On my current dinghy trailer where the lights get wet all the time, I keep the insides (sockets, connectors, etc) greased with silicone grease, white lithium grease or Boeshield. That seems to keep the corrosion at bay.
Lastly, you might be able to get or engineer LED trailer lights that could be encapsulated in epoxy. Since they don't get hot, they could be submerged ... this might work. Of course, the connections would still need to be made where they stay dry or keep 'em greased.
Good luck.
