Linda is right. It's likely a behavioral issue. Jealousy, fear, territorial rights, etc...
I'd be curious to know the adoption order (when this cat came into the group) as it would be telling.
to start, try giving it equal/extra attention. If it's jealousy, that could help.
If you go with the squirt gun, try to use it only in emergencies (serious violence). Trying to out stubborn a cat who's doing something that he/sh doesn't see as wrong can go badly for you. The cat doesn't understand why it's being "persecuted" and becomes resentful and angrier.
Also, if kitty doesn't know where these "Water attacks from God" are coming from, you'll likely drive kitty nuts and make him jumpy and paranoid. Since kitty most likely won't get the association of being squirted with water and what he's doing...it will just think the cat God's are whimsical in their wrath. :)
I have had the same problem....well, first, I can suggest getting the male cat neutered. But, that didn't fix my cat. He has always ambushed the other cats in the house. He gives a look-from-hell and goes after them. Not as much now as when he was younger, but still enough to upset the house. I read books, even got a cat psychic in.....but the bottom line, I think, is that he is jealous, protecting his property and area, and most importantly, his owner. He is very jealous when he thinks another cat will have my attention. One thing I tried earlier on, was to get a apray bottne full of water, and when he got his mean look, spray him, making sure he didn;t see me to know it was me,. Well, he outsmarted me, then I reeally looked like an idiot as one time, I was so mad at him I chased him around the house spraying him!!!!!!!!!! It didn;t phase him, but he thought I was loony! But, this also doesn;t work when you are away and he attacks on his own,. I just learned to live with it, and put him in his own special room during the night, with water, food, bed, etc, so the other cats can have a break. Your cat is still very young, so perhaps he will grow out of it, as maybe now he is thinking it is play-fighting.