Volvo AQ130c

VOLVO PENTA AQ130C. I have a bad water leak coming from between the exhaust manifold and the engine block, can this be a blocked manifold ? Boat is out of the water and I have run a hose to the pump. While testing last month everything was fine, water was flowing nicely through the exhaust and no overheating issues. However now engine seems to run rough and no water is flowing through the exhaust. I have not run it long as I don't want to cause more harm. Can it be a less serious issue than a defunct manifold. I was thinking of firstly trying out a water pump rebuild and thermostat before buying a new manifold, any advice would be greatly appreciated, Regards Sean