I think guys mount them hear the shock mount. The XPL's have the same set up and actually have a bracket that mounts to it to hold a rule pump. Find out where you're water is coming from. I can sub the front of my HX and have the water come up over the bars and that is the only time I'll get water in the hull. Other than that it's pretty dry.
I'm not taking on too much water, but I would like one for reassurance when I'm a long swim away from the ramp. I probably don't need one, but I have one lying around so why not.