You can see them being trailered to lakes and rivers all over our country. What you may not notice, is how the boats are unloaded from the trailers. Usually the trailers are backed down a sloping boat launch until the boat is floating in the water. Then the boat can be moved off to the dock while the trailer and tow vehicle are parked. The problem is water gets into the wheel bearings on the trailer and the rear axle on the tow vehicle.
Water causes the metal bearings to rust, and the rust causes the bearings to fail.
Keeping the trailer wheel bearings out of the water can be difficult. Usually the wheels must be almost all the way under water before the boat will float off. The seals on the trailer axle are designed to keep out splashed water and dust, but have difficulty in stopping water when they are completely submerged. When inspecting the wheel bearings, you should wash them completely clean with a solvent and let them air dry. An even grey or frosted appearance indicates a good used bearing.