liberally spray, hose, and rinse down the trailer and especially the brakes with clean fresh water after salt water dunking as soon as possible after each dunk.
being it an annual event, spending an extra 10-15 minutes to wash/rinse the trailer seems like easy cheap maintenance. could even go so far as to also give the trailer a blow off with an air gun to remove most of the excess water around the brakes which could lead to problem, but IMO pretty minimal if fresh water.
alternatively, you could install a fresh water brake rinse system similar to those found on systems fitted for drum brakes. Then all you'd do is hook up the fresh water hose to the inlet and flush for a few good minutes and be done.
also consider cadmium or stainless brake components over the standard "rusty" steel components for longer life in the salt environment.