I doubt it is underwatering. When a plant is underwatered the leaves droop and then fall off. Your leaves are curling over. That sounds like a nut. defecency. A good possiblity is a potassium defecency. A organic way of fixing this imbalance is to add some wood ashes to your water. You can also buy chemical fertilizer with potash to help fix this as well. Hope I was some help.

Happy Toking :rasta: