Originally Posted by DanielF
but on the other hand there's the saying that there's "no such thing as a surface crack."
In the case of fibreglass, that saying would be wrong. Surface cracks in gelcoat are very common. They happen because the area is flexing, and gelcoat is not so flexible so it eventually cracks.
You can grind out the crack and fill with epoxy. To really stop it happening again, reinforce the back of the area to stop or reduce the flexing.