Bare spot on trailer wire causing a short or pinched wire causing a short
The fish have learned basic wiring skills and are sabotaging your ride in the hopes that you stay home!
If you have a loose ground wire or the little sockets inside the flat rubber plug are not tight on the part that goes inside them, you can blow a fuse. Intermittent contact can do it. Try squeezing the sockets in the flat rubber plug VERY GENTLY. This is a problem that usually blows the fuse after you drive a few miles.The brake lights are on different fuses.Good luck,Keith
I had the same thing happening with my truck and it happened to be one of the wires coming from a rear taillight assembly that was rubbing raw on the inside of the body.
It could also be water in the light housing.
Good point! I have one light housing that is broken and after bringing back out of the water perhaps water stays in there? (I do unplug before I launch).
LED marine trailer lights.https://www.amazon.com/CZC-AUTO-Submersible-Trailer-Snowmobile/dp/B06ZZRKYHD/ref=sr_1_1?s=automotive&ie=UTF8&qid=1503579701&sr=1-1-spons&keywords=led+trailer+lights&psc=1I will NEVER use incandescent lights again on any trailer I own. PERIOD!