I learned to ALWAYS unplug the trailer lights before launching/retrieving. It cost me $97.95 to learn that it was a fuse. I had a blinking ABS lite on the dash. Thought something was wrong with the brakes since I had just had them replaced. Then I noticed I had no brake lites nor 4 way flashers. I just knew it was something wrong with the brakes. The Ford Dealer did a diagnostic check at $97.95 and found that a fuse was blown. Hey, they didn't charge for the fuse tho.
Either unplug the lites or have the connector high enough so as not to get it wet. (Or carry extra fuses)