Output 61 with Eprom revision 2.06 and higher is now NON-Silenceable. To meet UL and NFPA 72 standards.
The workaround is to program all your Waterflow points with the same Output Zone (1 -50). Then assign output selected (1 - 50) to the NAC's that need to sound for a waterflow. For this to work the Point Function selected for Waterflow needs to be set to Silenceable (Yes).