We had a crack form in our 5 year old fiberglass pool, a structural crack that caused water loss continually. The following is what I did to repair the crack, as we couldn't get the manufacturer or contractor to do anything about it, and had gone for about 7 months (last fall to this spring) with the crack going untreated. Over the winter season, the water level leaked down to the crack and stayed there. Anyway, here is an account of the steps I took... I did some dumb things that I don't necessarily recommend, but hope it is helpful to someone.
http://pwp.att.net/p/PWP-poolrepair
http://pwp.att.net/p/PWP-poolrepair