It strikes me that you probably have a very mature child. Meaning that they don't WANT to be throwing a fit, and have found a way that "works" to stop it.
I have to say, I like the water idea. It strikes me as practical, and painless, and quick. Combine that with talking & showing other ways to calm down (taking big deep slow breaths, counting ...um my one year old could count, not that he really knew what the numbers MEANT at that age...but we counted when he got upset), etc. Whatever sorts of things you do with your children to teach them self control in hard situations.
So, too, once a person passes out...they DO start breathing again. I've only known of one child who didn't have the breathing reflex...and she stayed in the hospital until she was 6 years old (she had to be taught to breathe consciously...and would forget if she got excited or distracted). Since your wee one not only breathes but has already developed conscious control, I wouldn't worry.
BTW...have you started swim lessons yet? That could be another way to phase out the holding-breath thing. The "You hold your breath when you're underwater, not when you're in the air. In the air you breathe. Under the water, you don't."