sheard impeller shaft

[Striper Blues]

Yikes, I thought I finally had my 3.0L OMC cobra running smoothly when another disaster. I was out on the water and the engine started pinging, I looked down to see that it had overheated. I shut it down checked around and finally found that the shaft that drives the impeller pump on the out drive had sheared off???? The impeller was fine as was the oil level in the outdrive. What the flock...... From what I can see after a quick peek at the parts manual, I'm screwed as this shaft is actually part of the major u-joint assy in the upper unit. Some one tell me if you can what would cause this to break off almost cleanly? And am I really screwed on this, like major bill for overhaul of upper? Thanks