Fiberglass.......
If you want the cracks to go away permanently, they need to be ground out and glassed over. You also need to find out what caused them to begin with and cure the problem. If the cracks are in an inside radiused corner, chances are the deck is flexing and needs to be supported. If the cracks are near a bulkhead or the like, chances are the wood is fit too tightly to the hull and needs to be relieved. If the cracks are extending from screw holes, the holes need to be properly ground out, glassed up and then when the new ones are drilled, you should use a countersink bit to bevel the entrance. Anyways, hope this infor helps, Good Luck!