What are those idiots trying to accomplish by kicking him that lightly? That dude is ******* CRRRAAAZY. He's so persistent in his little stabs regardless of getting knocked around. He's a man on a mission.
Anyway you have two options:
1:Soccer kick the face like all of you guys said
or if you kick apparently as hard as a passive 5 year old
2:Armbar him from behind... gotta make sure to break the arm and take the knife away. Youre still not out of water yet because hes got crazy man strength. Hope for back up.