Is it morally permissable to deny myself food and water in a living will if I should become incapacitated?
You could state that you wish food and hydration to be removed if they fail to sustain your life or if they cause you physical injury – unfortunately, suffering is too broad a word in our day and age – but, morally speaking, you cannot deliberately deny yourself ordinary care, which includes food and water. Besides, starvation and dehydration cause a person to die a slow and extremely painful death. Why would you want to subject yourself to that?
For more information about preparing an advance medical directive and durable power of attorney that is in line with Catholic moral teaching, I recommend contacting the National Catholic Bioethics Center.