When you have the muffs on the drive without it running, there's a lot of 'overspill' of water. When you fire up the engine, does that overspill diminish? If it does, that means the pump is pulling water in, is there an immediate flow of water from the propeller hub? if yes, then you missed the tube (between the impeller housing and the water pocket cover in the drive shaft housing)...
If the water overspill doesn't decrease, then the impeller isn't turning, or there's an air leak in the suction side.
Chris.........