you can use the pre-existing "Spindle M-codes" to control your water jet. M3 could turn it on, and M5 could turn it off. You could user your Spindle speed value as a value (analog or digital), to control the water speed.
If you need other I/O for specific functions unique to a water machine, just make you M-Codes with VB script in them that turn on/off through the PP or Mod-bus, if you need to send other analog values send those through the mod bus as well.
Scott