Adam Clarke on Matthew 17:15

My son - is lunatic - Σεληνιαζεται. One who was most affected with this disorder at the change and full of the moon. See on Mat 4:24 (note). But this lunacy was occasioned by a demon, see Mat 17:18, and Mar 9:17; Luk 9:38. In this case, the devil intended to hide himself under the appearance of a natural disorder, that no supernatural means might be resorted to for his expulsion. See a remarkable account on Luk 9:39 (note).

Falleth ofttimes into the fire, and oft into the water - The paroxysms of his disorder frequently recurred; and among his numerous falls, some were into the fire and some into the water: so that, on this account, his life was in continual danger. Those who are under the influence of the devil are often driven to extremes in every thing. Such are often driven into the fire of presumption, or the waters of despair. Satan takes advantage of our natural temper, state of health, and outward circumstances, to plague and ruin our souls.