Where does the crack emanate from on the forward end. Sometimes there can be a flaw in the casting or a nick or gouge that can create a stress riser in the casting, and then a fatigue failure occurs as the crack progresses. If in fact, it was a casting flaw, depending on the year you may be able to get Yamaha to cover it. If you think it might be from freezing, expensive lesson learned. If there is no internal gear damage and no metallic contamination on the drain plug, then short of physical damage, or freezing damage, the most common cause would be a flaw in the casting.