Trailer parts are usually resistant to corrosion to some extent, but some take it up a notch. For the record, boat owners should worry more about salt than the water itself, especially if boating by the sea. It doesn't matter if you have the most robust axles money can buy; the salt will eat them away one atom at a time.
To counter this threat, manufacturers resort to dipping their parts in corrosion-resistant metals, such as zinc. This is called hot-dip galvanizing and it's commonly used in fabricating machine parts and building materials. To understand why zinc is often chosen for the task, it's important to know how corrosion does its corrupting magic.
Seawater corrodes metals in one of two ways: electrolysis and galvanic corrosion. Electrolysis involves a stray charge that passes through the conduit—the salt—and initiates oxidation. On the other hand, galvanic corrosion involves the exchange of charges between two metals with different relative activity.
Manufacturers know enough not to join two metals far apart from the galvanic scale, so it's safe to say that electrolysis is the bigger issue. Zinc protects the metal by reacting with water and air to form a patina. To maximize protection, hot-dip galvanizing calls for four anti-corrosion layers; the top layer being pure zinc and the rest an alloy of zinc and iron.
Galvanized trailer parts may or may not appear shiny, but this has no bearing on corrosion protection. It means the process may have been done differently.