On the inaugural voyage of the Freedom the CD told us that one of the main reasons the Freedom class is larger than the Voyager class is because of the weight and pumping equipment needed for the flow rider. If they tried to put one on the Mariner then a good bit of area below it would have to be "taken away" from it's current use.
But you never know. Marine engineers could always figure out a way to do it. But I can tell you that in order to get 30,000 gpm of water flow you need some good-sized pumps and they take up space.