Most states require that trailers be fitted with lights — taillights, stoplights, and turn signals, at a minimum.
You'll need to maintain electrical wiring that will be immersed in seawater. Just as wheel bearings heat up on the road and then create a vacuum when they hit the cold water, illuminated trailer lights generate heat, too. Most of the maneuvering — stop, turn, reverse — occurs at the ramp during launch and retrieval. When the hot housing hits cold water, it's just a matter of time before the seals around the assembly fail. Disconnect your lights in the staging area, a few minutes before launching if possible. This gives the tail light assemblies time to cool, which should help increase the life of the bulbs.
Upgrading to light-emitting diode (LED) tail lights can pay off in the long run as they have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, and smaller sizes.
Nothing kills electrical components faster than saltwater. Inspect lights before each use. Make sure the housings are waterproof for any trailer that will be immersed, but also know that "waterproof" is still no guarantee of a long life. Removable taillight assemblies avoid dousing these electrical components.
Depending on the complexity of the trailer's lights and other equipment (winch, brake controller, and so on), the wiring harness from the towing vehicle to the trailer might comprise anywhere between four and seven connectors. A four-way connector provides a ground circuit, plus three functions (typically left-turn/stop, right-turn/stop, and taillights). A fiveway connector includes all these, but adds a blue wire for a brake controller or auxiliary function. A seven-way connector includes all these, plus reverse.
A weak ground is a frequent cause of faulty lighting. It can manifest itself in a particularly vexing way when the lights work sometimes but not other times — say, when headlights are on. Be sure the ground on the tow vehicle is connected to an unpainted surface on the chassis. Also ensure that each light assembly is grounded to the trailer. Inspect all the wire runs along the trailer to be sure that it is supported frequently and protected from chafe against the trailer and abrasion from heavy road use.