When Schutzie went to the dark side and bought a SeaRay the trailer that came with it was, shall we say, well used and abused. He redid the brakes and lights, which caused him to become somewhat of an expert on what trailer lights need to be so they are "water resistant".
Understand, your lights are subjected to much more abuse just being on your trailer than being dunked for a few minutes. When he purchased replacement lights he had "water resistant" lights, at a considerable premium price. An old salt at the store showed him the difference between "water resistant" and "inexpensive" lights. The housing and light sockets were identical, cause see, most of them are made by one manufacturer. The difference was the connectors and that the more expensive lights had a truly water tight grommet where the wires went through the light housing.
So. Couple of options for you
1) Simply disconnect the lights before you back your trailer in. They'll be dry before you need them. The only issue here is remembering to unplug and plug in your lights.
2) Some care installing your new lights will pretty much eliminate any issues.
a)Mount your lights as high as you can and ideally to the rear of something that will deflect road spray. High on a fender for example.
b) Use a single run of wire from the light to the plug. Where you must splice, heat shrink the splice. Twice.
c) Whatever plug you use, make sure it has good seals and that the seal where the wires go in actually seals.
d) A bit of grease in the sockets, at splices, and on the terminals in the plug will greatly reduce corrosion.
Schutzie has almost no experience with anything but the old fashioned 1157 bulbs, (cause he's an old fart) but most of the failures he had with his trailer lights involved hot bulbs hitting cold water (they will break). In practice the housing is almost water tight; as long as the upper half of the housing is pretty much sealed, any water that gets in won't get to the bulb or socket because an air pocket forms in the upper half of the housing. The rest of the system is pretty bullet proof, provided you don't actually launch your trailer by accident. In which case, you have greater issues to address.
A greater concern is the brakes and bearings; if you regularly submerge them launching or recovering your boat you will need to become intimately familiar with your brakes and bearings and such. I found that at minimum, an annual detailed inspection was mandatory. Plus, it was a good excuse to drink beer and cuss.