Varnish will protect wood against the elements. And – where the wood is of good quality – enhance the natural beauty of the surface.
It’s important you only use the Thinners recommended for each application. Not doing so, or substituting other proprietary products, will result in an unsatisfactory finish.
A boat will need maintaining during the season to ensure it keeps its good appearance and sailing capabilities and the coating system is maintained. How much maintenance and care is needed will depend on the environment in which the boat is sailed.