Need a little more detail or some pictures.
Generally, spider cracks, or stress cracks, that are in the gel-coat only are not a big deal. Gel-coat is very hard and not flexible. Spider cracks usually occur when the underlying fiberglass flexes and causes the more rigid gel-coat to crack. This is common on hatches that do no have sufficient backing or areas around the cooler which does not have rigid foam backing.
Having said that, stress cracks that go into the fiberglass are not good. These cracks mean the underlying fiberglass is weak or has been over stressed. These can be caused by poor lay-up when the fiberglass was put into the mold, or from the boat flexing from excessive stress.
The most common cause for fiberglass cracking is incorrect trailer support, or constant pounding of the hull when underway.
You need to determine if the stress crack is in the gel-coat only, or does it go into the fiberglass as well. If it is just the gel-coat, then repair is a simple matter of grinding out the crack and patching the gel-coat, wet sanding it, then polishing.
If the crack goes into the fiberglass, you need to get a good fiberglass repair person to grind down the area to solid fiberglass, build up the area with fiberglass matting, and then repair the gel-coat. You also should determine the cause of the cracking.
