Yamaha L4 3.0 with coolant water blockage in stern drive

I'm in the process of restoring a 1990 Ebbtide 170 XS with a Yamaha L4 3.0 stern drive. I finally got the motor running today but discovered that there is no coolant water. By process of elimination, the problem seems to be some kind of blockage within the stern drive unit itself. I've disconnected the hose that runs from the transom mount to the seawater pump and the hose is bone dry although the flusher is supplying plenty of water. I've got all the Yamaha manuals, engine and stern drive, but there's nothing showing the coolant water path through the lower unit and transom mount so I have no idea where to look for the blockage. Does anybody have a clue as to what or where the blockage could be and do I need to remove the lower unit to access the coolant water path?