Most cracks found on fiberglass boats are gel coat cracks commonly known as crazing.
They can appear as a single line, a series of parallel lines, or a spider web originating at the center of an impact.
It is caused by an impact or stress with enough force to crack the surface, but not the glass beneath.
The gel coat forming the finish surface is pigmented resin about 15 thousands thick. Since there is no glass reinforcement in this surface, it is more susceptible to crazing than the fiberglass under it.