Spider cracking is a somewhat normal occurrence on boats and often occurs in areas where the gelcoat was put on too thick or allowed to "pool" and corner areas. As long as yours is not from hitting something, I wouldn't be too worried. 99% of the time they are strictly cosmetic. As noted above, they're not always the easiest thing to fix. While the underlying fiberglass is not directly affected by stress cracks, the repair does involve grinding down to glass and actually making the crack bigger before it can be fixed. My opinion: Just keep an eye on it and make sure the cracks don't WIDEN.