How are bridges and/or tunnels that go over or in water maintained?

In rare cases, they are constructed of corrosion-resistant materials. But for the most part, they are painted to limit and slow corrosion. Where corrosion does occur, the material is stripped down to the base material, cleaned of the corrosion, and repainted. The process is repeated over the life of the structure. The frequency is usually determined by inspection and the how hostile the environment turns out to be.