Yeah I agree with fragmeister start at the earth and make sure your vehicle is earthed correctly also buy a test light and check your plug and see how it's all working at the vehicle that way you can isolate the problem to an area Vehicle /Trailer.
Now as for the trailer issue unless the boat is loaded more to one side to upset the balance seem to be a weak spring or a roller may have slipped etc just check all the load contact points and good luck with the fishing tomorrow .