Here is a diagram of the path of water flow thru the engine.
Sail Drive Cooling
If you are saying that you have waterflow in the hose labeled "out" up to the point where it connects to the exhaust riser, then it would seem that the blockage is in the riser and extending that hose overboard might work.
Becareful though because that might create an exhaust leak at the point where the hose is removed from the riser.
Some folks have had problems with exhaust riser blockages caused by engine coking.