Water
This is totally normal. When your using the boat, water comes from inside the jet pump, thru the exhaust system and then out the flush fitting. Water is running in reverse when you are flushing......going in the flush fitting and coming out the center of the pump. You should ALWAYS have water coming out the center of the pump when flushing.....if not then you have a clog in a hose, the pump or a hose has come disconnected inside.
DAWG