Gel-coat, the exterior finish on fiberglass boats, is a porous mix of polyester resins and pigments that must be sealed from the damaging effects of salt, dirt, weather, and ultra-violet (UV) radiation. 

The primary purpose of gel-coat is to protect the underlying core from water, air and sunlight, which in combination will quickly loosen the chemical bond and render the structure useless.  Neglect this gel-coat, and it becomes more porous, resins oxidize and what remains is a faded, dull finish prone to absorbing water into the laminate and destroying it.

In addition to the above functional definition we also need to say a few words about the beauty factor, the shine of a newly polishedand waxed boat.  In a great example of form following function, gel-coat kept clean and shiny, with a good coat of wax, will perform it's primary function pretty much in direct proportion to the way it looks.