Yes, its used as reaction mass

And Mr Ritter's quote of 4 to 1 water to ship mass is only for the most economical use of the anti-matter. AM is very costly stuff to produce so you want to minimize it if you can and use more water. But if you use more water than 4 to 1 the temperture of the water is lower as is the exit speed of the reaction mass.
You could use less water for a lighter ship but you run into other problems.

Galen: Hmm? Oh, sorry. I was just trying to work out whether a comment from me right now would be 'most wanted' or 'least needed'.
Expect me when you see me.