Find cause
Well, first, determine what might have caused it. Stress...Did something bang into your boat...whatever. If it is spiderweb crazing of the gelcoat, then it's not a structural problem per se. Gelcoat is purely a cosmetic material lathered on about 1/8th inch thick in the mold before they lay up the fiberglas. It is to fiberglas what the candy-coating shell is to M&Ms. Just for looks more than anything else. But, it can start to eventually seep, allowing water to get to the glass and then it'll just crack right off. The fix is easy; just some barrier coat fillet, sanding and painting and you're done. As to what to do about it at this point, well, it kinda depends on how much work you want to do and how deep the problem is. Is it surface? Then just keep an eyeball on it and keep it filled with a good wax. Is it a crack that goes below the surface? Well, a possible fix -- and there could be thousands -- is to use a Dremel moto-tool with a small V-notch router bit and a guide and burrow out a small notch the length of crack, fill the crack with expoxy that you make rather stiff so it doesn't sag and lay it up inside the notch. It'll give a good seal and won't leak. Might not look the best, but it'll do til you decide to repaint the boat. This is just one suggestion and there could be plenty more. Don Casey has a very good series of How-To books out that are generally priced about $21/copy and most book and marine stores carry them. He has one out on deck and hull repair and that might be a good place to start for some more information and some well-illustrated how-to info. Good luck.
